[FTS] explicitly invoke GC when over memory quota

Related to these other tickets MB-24954 (throttle http requests if using too much memory) and MB-24823 (high memory usage observed in system test), here's another idea... explicitly invoke golang's runtime GC when we're over memory quota.

This is a technique that GSI/secondary-indexing team uses.

Need to follow-up with Deepkaran Salooja or other indexing colleagues to see if this kind of approach works well enough or is/isn't worth the effort.