Indexing: clear versionMap on refresh (not flush) to reduce heap usage

The versionMap holds all versions (keyed by _uid) for recently indexed
documents.  Previously we only cleared it during flush, which can be
infrequent if the translog flush thresholds are high, and can cause
excessive heap usage especially for small documents.

Now we clear it during refresh which is usually more frequent
(e.g. once per second by default).

Closes #6379