Some optimizations, including debug code.
Temporarily disabled unsaved change throttling
driver/python: silient unfriendly tracebacks

Qutoes comments by coffeemug from issue #2098:

  I noticed from #2097 that some user errors are printed as Python
  exceptions (with Traceback and everything). They shouldn't be -- we
  should just print proper error messages and return user values. Printing
  Python exceptions and tracebacks is really unfriendly in those cases.

Link: https://github.com/rethinkdb/rethinkdb/issues/2098

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>
Do not extend Array.prototype
Merge branch 'next' into daniel_2071

Conflicts:
	src/buffer_cache/alt/page_cache.cc
Process backfills one key-value at a time, and release locks earlier when receiving backfills.
Reverted temporary debugging code
Remove redundant code.
Revert "Remove redundant code."

This reverts commit 6e7677f666da0a21c44fbb101c2d791014646bb4.
Undid the single kv-pair backfill chunk change.
Optimized sindex queue push (and release sindex block earlier during backfilling)
Tuned a chunk size, and releasing sindex block earlier during backfills.
Made chunk size a bit adaptable to chunk_factor.
adding cache_size parameter to machine directory metadata
Merge pull request #2111 from Aleaxander/yliu-2098

driver/python: silient unfriendly tracebacks
Merge branch 'michel_arrayresult' into next
Added constness
Merge branch 'daniel_2071' into next
Merge branch 'next' into sam_multiple_index_writes
Merge branch 'sam_multiple_index_writes' into sam_multiple_iii_better