Added an extent_size perfmon.
Reduced header inclusion by perfmon headers.
Made combine_stats take a const argument.
Renamed make_scoped_ptr to make_scoped.
Made perfmon_t::end_stats return a scoped_ptr_t<perfmon_result_t>.
Made perfmon_filter_t::subfilter innards be a bit more hygienic.
Made perfmon filter and subfilter functions use a scoped_ptr_t<perfmon_result_t>.
Made perfmon_filter_t::subfilter not spuriously use std::list.
Fixed at-least-sometimes-wrong leave-behind-dead-map-nodes logic of perfmon_filter_t::subfilter.

Some map values were getting set to NULL, but then those were not
getting deleted, thanks to a premature return statement.
Made perfmon_result_t::internal_map_t use a scoped_ptr<perfmon_result_t>.
Made perfmon_perthread_t::output_stat use a scoped_ptr_t.
Fixed last worthy scoped_ptrification of perfmon_result_t.
Merge branch 'next' into sam_310
Removed unused pm_extent_size perfmon that was not even added to a collection.
Made perfmon/archive.cc compile in debug mode.
Merge branch 'next' of github.com:rethinkdb/rethinkdb into next