Move an assertion to the top of a function.

This is in response to defect 626. And is mostly a stylistic change.
Remove an unneeded declaration.

In response to defect 647.
Remove some forward declarations.

They're no longer needed because everything in btree_store_t is public
these days and they were being used for friending purposes.

This is in response to defect 573.
Remove a TODO which is done.

This addressses defect 574.
Remove a commented out private specifier.

This addresses defect 575.
Fixes indentation on some methods.

This addresses defect 648.
Removes an unneeded public specifier.

Everything in btree_store_t is public so it isn't needed.

This addresses defect 576.
Remove a commented out private.

Addresses defect 649.
Switch a reinterpret_cast to a safer static_cast.

This addresses D650.
Remove a field used for debugging.

Addresses D578.
Explicitly compare a pointer to NULL.

Addresses D651.
Relocate some includes.

Addresses D579.
Remove an UNUSED specifier in a virtual function.

Addresses D580.
Makes include guards follow our style.

Addresses D653.
Comment and #endif.

Addresses D652.
Check for default constructed rdb_modification_report_ts.

This addresses D583.
Removes an unused function.
Change a throw specifier.

Addresses D584.
post_construct_and_drain_queue can't throw anymore.

It used to be able to throw interrupted_exc_t this was bad because it
was spawned in its own coro which meant it could crash the server. Now
it can't.

This addresses D585.
Check interruptor in queue drain loop.

Addresses D586.