space: introduce space_execute_dml helper

I'm planning to call BEFORE triggers in space.c. Since a BEFORE trigger
can change the request type, we can't call it from functions handling
particular kinds of requests (space_execute_replace() and others).
So let's move the switch-case statement that executes different space
callbacks depending on the request type from process_rw() to a new
function, space_execute_dml(), defined in space.c. We will execute
BEFORE triggers from this new function, right before dispatching the
request by its type.

Needed for #2993