[LSan] Add new experimental option for checking long-running processes.

[LSan] Add new experimental option for checking long-running processes.

New LSan option: leak_check_interval_s:
Spawns a background thread at startup which periodically do recoverable
leak check with given interval.

Report files (if log_path option != stderr|stdout) are appended with
prefix "in-progress" and for the DoLeakCheck() call at exit -
are restores back.

System daemons can be leak sanitized right after system startup by adding
"Environment=LSAN_OPTIONS=...:leak_check_interval_s=60"
field in the systemd service file for leak checking every 60 seconds.

Change-Id: Ia78044ec45092f0de96818bd0114d4c7a49cb7e1