add several configuration options for Pregel jobs

[jsteemann]

docs PR for arangodb/arangodb#16332

[markuspf]

LGTM (mod typos) as general documentation;

At some point we might need to write sizing-guidelines or put more detail in places.

For example it is clear to me that the max/min parallelism settings limit each individual pregel job; this does not limit how many cores might be used by the pregel subsystem as a whole.

Also memory mapped files do not cause any overhead as long as data still fits in ram, because nothing is actually written to disk, but as soon as data does go to disk this has performance implications. (the extent of which we cannot give right now because we don't know)

[ansoboleva]

reviewed