Remove trx_t::active_trans. Split into two separate fields with distinct
responsibilities. trx_t::is_registered and trx_t::owns_prepare_mutex.
There are wrapper functions for using this fields in ha_innodb.cc. The
wrapper functions check for invariants.

Fix some formatting to conform to InnoDB guidelines.

Remove innobase_register_stmt() and innobase_register_trx_and_stmt().

Add:
trx_is_started()
trx_deregister_from_2pc()
trx_register_for_2pc()
trx_is_registered_for_2pc()
trx_owns_prepare_commit_mutex_set()
trx_has_prepare_commit_mutex()

rb://479, Approved by Jimmy Yang.