[libFuzzer] Periodically purge allocator's quarantine to prolong fuzzing sessions.

Summary:
Fuzzing targets that allocate/deallocate a lot of memory tend to consume
a lot of RSS when ASan quarantine is enabled. Purging quarantine between
iterations and returning memory to OS keeps RSS down and should not
reduce the quarantine effectiveness provided the fuzz target does not
preserve state between iterations (in this case this feature can be turned off).

Based on D39153.

Reviewers: vitalybuka

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39155

llvm-svn: 316382