helper functions that initialize some members of the transaction struct.
(Bug #41680)

innobase_trx_init(): New function: initialize some fields of a
transaction struct from a MySQL THD object.

innobase_trx_allocate(): New function: allocate and initialize a
transaction struct.

check_trx_exists(): Use the above two functions.

ha_innobase::delete_table(), ha_innobase::rename_table(),
ha_innobase::add_index(), ha_innobase::final_drop_index():
Use innobase_trx_allocate().

innobase_drop_database(): In the Windows plugin, initialize the trx_t
specially, because the THD is not available.  Otherwise, use
innobase_trx_allocate().

rb://69 accepted by Heikki Tuuri