Cleaning up msat_terms, at least optionally

[cheshire]

Currently, we never deallocate msat_terms.
Mathsat5 has a function msat_gc_env, which can be called periodically for this purpose.
This can be enabled at least as an option.

[cheshire]

This is more difficult than I've thought. msat_gc_env is very expensive, and we currently don't have a notifier which would say "ooops we are about to run out of memory". Calling GC on every allocation is prohibitive.