Make sure that RecoveryUnit::commit() can safely throw WriteConflictException

Our first implementation of CC in RocksDB was to resolute conflicts in the commit() call. However, there are two things that didn't work:
1) Some commit() calls don't catch WriteConflictException. One example is db/commands/mr.cpp
2) Mongo expects commit() to succeed because of https://jira.mongodb.org/browse/SERVER-15192

We're currently transitioning our CC implementation to check conflicts on write rather than commit. However, in the future we might want to transition back to commit-time-resolution.