Removed locally used helper function declaration from depth_first_traversal.hpp.
Made handle_pair take a dft_value_t, added movable_t and counted_buf_lock_t.
Added a btree_concurrent_traversal stub implementation, that lacks real concurrency.
Holy crap, it works.  Made us load data concurrently off of disk when doing an rget.  Have not run any benchmarks so far.
Made concurrent btree traversal use an adjustable semaphore.
Actually initialized sink_waiters_.
Renamed dft_value_t to scoped_key_value_t.
Added a useful comment about scoped_key_value_t.
Fixed return value behavior of btree_concurrent_traversal.
Moved last_considered_key update into the exclusive zone.
Ostensibly adds tagged indexes.

It's untested right now. Next up is driver support.
Renamed concurrent_traversal_waiter_t to concurrent_traversal_fifo_enforcer_signal_t.
Merge branch 'v1.8.x' into sam_alt_1296
Tagged indexes are now passing cursory tests.
Merge branch 'sam_alt_1296' into sam_merged_1296 (dropping efficient count()).

Conflicts:
	src/rdb_protocol/btree.cc
Fixes several bugs in tagged indexes.

They now seem to work correctly.
Replace tags with multi. Support in rb and js.

RB and JS pass smoke tests now.
Adds a gitignore.
Fix a bug in python driver.
Fix some regression tests that were made incorrect.