Fixes Bug #530870
* in drizzled/lock.cc, the involved_engines calculation was incorrectly
using getSlot() instead of the new unified MonitoredInTransaction::getId()
method for determining uniqueness.
* Removed InnoDB's trx_t's active_trans data member. Now using only
trx_t's conc_state member to determine state of the transaction (imagine that.)
Added asserts() to test some assumptions about when and how a trx_t's
conc_state can and cannot be in TRX_NOT_STARTED state.
* Remove getSlot() from plugin::StorageEngine.