ThreadID printed when Thread terminating in the same format as posix_logger

Summary: https://github.com/facebook/rocksdb/commit/220132b65ec17abb037d3e79d5abf6ca8d797b96 correctly fixed the issue of thread ID printing when terminating a thread. Nothing wrong with it. This diff prints the ID in the same way as in PosixLogger::logv() so that users can be more easily to correlates them.

Test Plan: run env_test and make sure it prints correctly.

Reviewers: igor, haobo, ljin, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D18819