sql: rework triggerStepAllocate to set diag

Refactored triggerSterAllocate routine as sql_trigger_step_new
and reworked it to set diag_set in case of memory allocation
error. Also performed some additional name refactoring in
adjacent places.
This change is necessary because the sql_trigger_step_allocate
body has a sqlNormalizeName call that will be changed in
subsequent patches.

Part of #3931