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.
Renamed concurrent_traversal_waiter_t to concurrent_traversal_fifo_enforcer_signal_t.
Merge branch 'v1.8.x' into sam_alt_1296
Merge branch 'sam_alt_1296' into sam_merged_1296 (dropping efficient count()).

Conflicts:
	src/rdb_protocol/btree.cc
Merge branch 'next' into sam_merged_1296
Merge branch 'sam_1426' into sam_1400
Made validate_pb specially validate VAR terms.
Reverts part of commit c40066173f4f536b6fa06127d4d38c5c1b33fe63 -- the part involving changing datum_t::cmp.

Conflicts:

	src/rdb_protocol/datum.cc
Used std::string::compare, wrapped in signum functions.
Removed signum wrapping, made users of cmp functions handle the return value correctly.
Undid removal of "!= 0" that happened in the recent reversion of datum_t::cmp changes.