accounts/keystore: use map instead of slice to keep all accounts

[AlexanderYastrebov]

After profiling geth startup times for #16874
it turned out that most of the time was spent resizing accounts slice
for ordered insertion.

This change uses map to track all accounts instead of slice to improve performance.

geth startup time using keystore with one million accounts reduced from
23 minutes to 2.5 minutes.

Benchmarks show relatively-small overhead increase for small keystores and large
decrease for huge:

goos: linux goarch: amd64 pkg: github.com/ethereum/go-ethereum/accounts/keystore │ HEAD~1 │ HEAD │ │ sec/op │ sec/op vs base │ Add/preload=10-8 1.030µ ± 5% 1.447µ ± 7% +40.51% (p=0.000 n=10) Add/preload=100-8 1.109µ ± 4% 1.463µ ± 3% +31.88% (p=0.000 n=10) Add/preload=1000-8 1.860µ ± 3% 1.477µ ± 5% -20.57% (p=0.000 n=10) Add/preload=1000000-8 5177.640µ ± 2% 1.654µ ± 11% -99.97% (p=0.000 n=10) Find/preload=10/by_address-8 23.70n ± 1% 23.88n ± 3% ~ (p=0.271 n=10) Find/preload=10/by_path-8 50.43n ± 2% 39.88n ± 5% -20.94% (p=0.000 n=10) Find/preload=10/ambiguous-8 323.6n ± 1% 1049.0n ± 3% +224.17% (p=0.000 n=10) Find/preload=100/by_address-8 23.69n ± 1% 23.63n ± 6% ~ (p=0.739 n=10) Find/preload=100/by_path-8 362.70n ± 1% 37.84n ± 3% -89.57% (p=0.000 n=10) Find/preload=100/ambiguous-8 2.683µ ± 2% 19.235µ ± 2% +617.05% (p=0.000 n=10) Find/preload=1000/by_address-8 26.45n ± 1% 27.73n ± 2% +4.82% (p=0.000 n=10) Find/preload=1000/by_path-8 3211.00n ± 3% 38.22n ± 8% -98.81% (p=0.000 n=10) Find/preload=1000/ambiguous-8 26.14µ ± 2% 263.59µ ± 1% +908.41% (p=0.000 n=10) Find/preload=1000000/by_address-8 26.47n ± 4% 26.41n ± 1% ~ (p=0.566 n=10) Find/preload=1000000/by_path-8 3683325.50n ± 4% 44.09n ± 45% -100.00% (p=0.000 n=10) Find/preload=1000000/ambiguous-8 39.68m ± 14% 819.48m ± 7% +1965.01% (p=0.000 n=10) geomean 2.346µ 791.4n -66.27% │ HEAD~1 │ HEAD │ │ B/op │ B/op vs base │ Add/preload=10-8 643.0 ± 0% 662.0 ± 0% +2.95% (p=0.000 n=10) Add/preload=100-8 643.0 ± 0% 662.0 ± 0% +2.95% (p=0.000 n=10) Add/preload=1000-8 584.0 ± 5% 662.0 ± 0% +13.36% (p=0.000 n=10) Add/preload=1000000-8 88.00 ± 0% 662.00 ± 17% +652.27% (p=0.000 n=10) Find/preload=10/by_address-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=10/by_path-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=10/ambiguous-8 624.0 ± 0% 1200.0 ± 0% +92.31% (p=0.000 n=10) Find/preload=100/by_address-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=100/by_path-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=100/ambiguous-8 6.047Ki ± 0% 12.047Ki ± 0% +99.22% (p=0.000 n=10) Find/preload=1000/by_address-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=1000/by_path-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=1000/ambiguous-8 56.05Ki ± 0% 112.05Ki ± 0% +99.92% (p=0.000 n=10) Find/preload=1000000/by_address-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=1000000/by_path-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=1000000/ambiguous-8 53.41Mi ± 0% 106.81Mi ± 0% +100.00% (p=0.000 n=10) geomean ² +36.09% ² ¹ all samples are equal ² summaries must be >0 to compute geomean │ HEAD~1 │ HEAD │ │ allocs/op │ allocs/op vs base │ Add/preload=10-8 3.000 ± 0% 4.000 ± 0% +33.33% (p=0.000 n=10) Add/preload=100-8 3.000 ± 0% 4.000 ± 0% +33.33% (p=0.000 n=10) Add/preload=1000-8 3.000 ± 0% 4.000 ± 0% +33.33% (p=0.000 n=10) Add/preload=1000000-8 2.000 ± 0% 4.000 ± 0% +100.00% (p=0.000 n=10) Find/preload=10/by_address-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=10/by_path-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=10/ambiguous-8 2.000 ± 0% 3.000 ± 0% +50.00% (p=0.000 n=10) Find/preload=100/by_address-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=100/by_path-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=100/ambiguous-8 2.000 ± 0% 3.000 ± 0% +50.00% (p=0.000 n=10) Find/preload=1000/by_address-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=1000/by_path-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=1000/ambiguous-8 2.000 ± 0% 3.000 ± 0% +50.00% (p=0.000 n=10) Find/preload=1000000/by_address-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=1000000/by_path-8 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹ Find/preload=1000000/ambiguous-8 2.000 ± 0% 3.000 ± 0% +50.00% (p=0.000 n=10) geomean ² +21.97% ² ¹ all samples are equal ² summaries must be >0 to compute geomean 

Updates #16874