Preparations for replacing rdb_erase_major_range().
Replaced rdb_major_erase and removed some related complications. Compiles except for unit tests.
Merge branch 'next' into daniel_2329

Conflicts:
	src/rdb_protocol/btree.cc
	src/rdb_protocol/store.cc
	src/unittest/rdb_btree.cc
Dedupped one of the functions that updates secondary indexes. Fixed the unit test.
Fixed order tokens in reset_data()
Made erase_range callbacks use enum types. Plus other minor stylistic fixes.
Move metainfo update out of reset_data().
Merge branch 'next' into daniel_2329

Conflicts:
	src/rdb_protocol/btree.cc
	src/rdb_protocol/btree.hpp
	src/rdb_protocol/protocol.cc
	src/rdb_protocol/store.cc