Added some RSI comments about env_t fields concerning parallelized evaluation.
Removed a performance TODO that seems to be largely resolved for practical purposes.
Made env_t::extproc pool (the field, not the pointee) be const.
Made env_t::trace be const.
Removed pointless ns_metadata_t typedef.
Changed a std::bind back to a boost::bind to avoid scope creep.
Removed env_t::throw_if_interruptor_pulsed (which was used only twice).
Adjusted comment about semilattice_metadata.
Clarified a comment.
Merge branch 'next' into sam_2156
Merge branch 'next' into sam_2156
Merge branch 'next' into sam_2156
Added DISABLE_COPYING to trace_t.
Made trace_t::as_datum be const.
Made disabled_ref_count be disabled_ref_count_ to be consistent with fellow fields.
Merge branch 'next' into sam_2156

Conflicts:
	src/rdb_protocol/env.cc
Merge branch 'next' into sam_2156
Moved evals_since_yield _before_ the coro_t::yield in env_t::maybe_yield.
Commented the env_t constructor that takes an rdb_context_t.
Commented another env_t constructor.