Summary:
Fixed unit test c_test by initializing logger=NULL.
Removed "atomic" from last_log_ts so that unit tests do not require C11 compiler.
Anyway, last_log_ts is mostly used for logging, so it is ok if it is loosely
accurate.
Test Plan: run c_test
Reviewers: heyongqiang
Reviewed By: heyongqiang
Differential Revision: https://reviews.facebook.net/D4803