Began changing sindex deletion to be more robust.
Implementation of two-stage sindex deletion. Compiles, but still a little incomplete.
Bumped serializer version and proceeded converting the sindex deletion code.
Added a separate being_deleted flag that's independent of post_construction_complete. This is important because post constructed deleted sindexes must still be updated, while non-post constructed deleted sindexes must not.
Fixed last outstanding TODO!. Still untested and probably has some bugs.
Improved error message and fixed a block acquisition problem.
Fixed secondary_index_t::is_ready().
Delete sindexes properly, not leaking blocks.
Filter deleting sindexes from the sindex_list result.
rdb_protocol sindex tests now wait for sindex readyness (instead of doing a nap(100)).
Added a repeated sindex create/drop unit test.
Merge branch 'next' into daniel_2251 (part 1)

Conflicts:
	src/rdb_protocol/btree_store.cc
	src/rdb_protocol/protocol.cc
	src/rdb_protocol/store.hpp
	src/unittest/btree_sindex.cc
	src/unittest/dummy_namespace_interface.hpp
	src/unittest/dummy_protocol_namespace_interface.cc
	src/unittest/memcached_protocol.cc
	src/unittest/rdb_btree.cc
	src/unittest/rdb_protocol.cc
Fix build after previous merge (part 2 of the merge)
Merge branch 'next' into daniel_2251
Merge branch 'next' into daniel_2251

Conflicts:
	src/btree/erase_range.cc
	src/btree/secondary_operations.cc
	src/rdb_protocol/btree_store.cc
	src/rdb_protocol/protocol.cc
	src/rdb_protocol/store.cc
	src/rdb_protocol/store.hpp
	src/unittest/btree_sindex.cc
	src/unittest/rdb_btree.cc
Made and optional argument to acquire_sindex_superblock_for_* mandatory
Changed a buf_lock_t&& argument to a buf_lock_t argument.
Added an assert that triggers in debug mode if a deleted secondary index is accessed.
Slightly improved sindex unit test and cosmetic changes.
Refactored delayed sindex clearing. Added some TODOs.