client: UserMappingGrid: Fixed two race conditions which resulted in

flaky unit tests (mostly when run through Valgrind).

1st race condition: When the gridmap file was rewritten, it was
truncated first and then the new content was written. However, between
both modifications the "mtime" of the file did not change and therefore
no reload was triggered. To solve this problem, the file size is
monitored now as well.

2nd race condition: It was possible that a new version of the gridmap
file was written after the initial Start) and before the monitor thread
was started and initialized the first known date and size of the file.
Consequently, the monitor thread observed the new file as the latest
known file and could not observe the change of state from old to new.
Fixed by initializing monitored time and size of the gridmap file before
the monitor thread gets started.