fixing use of SimpleQueue on python3.4
Fix for `slice_datum_stream_t` logic so that `batchspec_t::all` actually reads everything.  (Also, minor performance tweaks to scaledown logic and some cleanup.)
Cleanup, rasserts to guarantees, and fix for changefeed::feed_t bug.
fix for #3219  build errors on Yosemite version of clang

over-the-shoulder by @danielmewes
removing DEBUG_VAR from auto_drainer_t::lock_t::assert_is_holding as this is used in production as well

over-the-shoulder and advice by @Tryneus
fixing alignment of object_buffer_t data
Silenced complaints about fabs calls implicitly converting a long double to double -- made the conversions explicit.
Merge branch 'v1.15.x' into next, fixing apparently incorrectly cherry-picked changes because why would anybody let the computer do work for them when you could do it manually and badly?

Conflicts:
	src/concurrency/auto_drainer.cc
	src/rdb_protocol/batching.cc
	src/rdb_protocol/datum_stream.cc
Merge branch 'sam_3202' into sam_fix_build