Made sync_callback_t inherit from cond_t privately.
Made sync_callback_t have increase_necessary_sync_count.
Made sindex_queue_push just use a single sync_callback_t with increase_necessary_sync_count.
Made some proof-of-acquisition params be const in btree_store.
fix clang compile error from unused variables on osx
Merge pull request #608 from taybin/osx_compile_fix

fix clang compile error from unused variables on osx

Reviewed by jdoliner.
adding sindex_list support to rdb_protocol
Merge branch 'sam_207' of github.com:rethinkdb/rethinkdb into sam_207
Merge branch 'next' into sam_207