Fixed `.between.distinct`.
Added tests for `.between.distinct`.
Added more tests.
Merged.
Improved process_headers.pl and fixed newly-found include weirdness in s2.
Fixed process_timings_output.pl.
Merge branch 'sam_process_headers_improvement' into sam_2134_base
Made log_serializer_t::unregister_read_ahead_cb disallow spurious unregister operations.
Removed default expected_change_count parameter in txn_t constructor.

Only a unit test used it.

Conflicts:
	src/unittest/new_leaf_node_test.cc
Removed some KSI comments.  Being more fatalistic.
Added and made use of half_intrusive_list_t.
Made some unit tests use distant_past instead of invalid.
Removed an RSI instructing me to remove commented out code (in changefeed.cc).
Removed fifo enforcer peanut gallery KSIs.
Renamed read_token_pair_t and write_token_pair_t to read_token_t and write_token_t.
Removed no-context KSIs about how the alt cache interface should change.
Removed KSI about renaming new_semaphore_t.
Changed a KSI to a TODO.
Removed some dumb KSI comments in operations.cc.
Removed a whiny KSI comment.