Addresses #1736, #1398 When inside straddle_hack callback don't use normal rwlock_read_lock for get_and_pin, but simply increment reader count instead. This prevents deadlock where callback waits for end_checkpoint to release write lock, but while end_checkpoint waits for callback to release read lock.

git-svn-id: file:///svn/toku/tokudb@11658 c7de825b-a66e-492c-adef-691d508d4ae1