DEL command implemented

Modified our MSET handler and made it generic (it has the same
general mechanism as DEL, just with a different return type)
and implemented the DEL command.

We've got the same consideration to make.  What to do in the
event of a catastrophic failure.
Return getThis() pointer from MULTI
Refactoring, updated handling of MULTI commands

Minor formatting changes

Updated how we delete context variables, such that our context
pointers aren't always going to be a zval** array.  This
introduces the possibility of creating a memory leak if we have
to abort a transaction due to a communications error, but this
can be handled later by attaching a void dtor(void*) callback.