gstlal-inspiral, gstlal-burst: port to dbtables.workingcopy

- previous versions of the ligo.lw library provided module-level functions for manipulating temporary database working copies.  these have been replaced with a context manager based mechanism.  these codes have previously been partially future-proofed in preparation for the conversion, and now that the new python-ligo-lw release is in production the rest of the conversion can be completed.
- NOTE: this patch does not update gstlal-inspiral/bin/gstlal_inspiral_coinc_extractor_dag nor gstlal-inspiral/bin/gstlal_inspiral_destagger_injections.  those two codes are missing the required clean-up function calls, and leak working copies out into scratch disks.  since I don't know what behaviour was intended, I can't correctly migrate them to the new mechanism.  their authors can use this patch as a reference for the use of the context manager when updating them.