Initialize atomic variable waitStart in PGPROC, at postmaster startup.

 Initialize atomic variable waitStart in PGPROC, at postmaster startup.
 
 Commit 46d6e5f567 added the atomic variable "waitStart" into PGPROC struct,
 to store the time at which wait for lock acquisition started. Previously
 this variable was initialized every time each backend started. Instead,
 this commit makes postmaster initialize it at the startup, to ensure that
 the variable should be initialized before any use of it.
 
 This commit also moves the code to initialize "waitStart" variable for
 prepare transaction, from TwoPhaseGetDummyProc() to MarkAsPreparingGuts().
 Because MarkAsPreparingGuts() is more proper place to do that since
 it initializes other PGPROC variables.
 
 Author: Fujii Masao
 Reviewed-by: Atsushi Torikoshi
 Discussion: https://postgr.es/m/1df88660-6f08-cc6e-b7e2-f85296a2bdab@oss.nttdata.com