Explore: Automatic re-prepare when...

This improvement is to discuss and explore if there is a way for us to automatically re-prepare (i.e. invalidate prepared cache entry plans) either based on plan attributes or configuration.

If for some reason there is an issue with index availability when a statement is prepared, it can be left using a valid but sub-optimal plan without DBA intervention.  For example, it may be planned to use a sequential scan (correct for the time of planning) and won't change even when a suitable index is later made available, without manually forcing a repeat of the prepare.

Two ideas thus far are:

1. Always mark plans with primary scans (regardless of indexer) as invalidated so they're plans are built on every execution.
   1. Down-side is that effectively we do away with prepared statements (the benefits thereof) for any primary-scan containing plan with no means to selectively avoid this.
   2. Would likely need feature control.
2. Introduce the concept of a TTL for plans after which they're automatically invalidated and rebuilt.
   1. TTL would be a cluster configuration defaulting to "infinite" to maintain compatibility with current code.
      1. Would therefore not need feature control.
   2. Would apply to all statements.