Made secondary_index_t be versioned.
Initialized reql version fields in the secondary_index_t default constructor.
Added a TODO comment.
Added a cluster_version_t parameter to the env_t, but did not pass correct values everywhere.
Removed wrong RSI comment.
Made secondary index env_ts actually receive the appropriate reql version.
Disallowed opaque_definition_out from being NULL in acquire_sindex_superblock_for_read.
Dropped pointless reql version parameter of non-sindex env_t constructor.
Turned an RSI into a TODO.
Improved the commenting around opaque_definition_t.
Removed the unpopular opaque_definition_t typedef.
Initialized secondary_index_t reql version fields properly in store_t::add_sindex.
Made the reql version fields be part of the opaque_definition, instead of the secondary_index_t.

This keeps all of the behavioral "sindex description" in the
opaque_definition.
Made cluster_version_t serialization only be exposed through serialize_cluster_version, again (because reql versions are now inside of opaque_definition).
Merge branch 'sam_2774_disk_structure' into next