Collection flush

Collections can be created, dropped and flushed, KV has no support for flushing.

Flush is to be first initiated by a set_manifest adding a "flush" tag to a collection-entry (see design for details https://docs.google.com/document/d/1X-v8GWQjplrMMaYwwWOzEuP4AUoDNIAvS39NmEjQ3_E/edit#heading=h.8c7d3zbqy7i)

A flush will work by moving the start-seqno of the CID so that any items with a seqno below the flush seqno become logically deleted.

The hash-table needs a recently removed patch re-instating so that the hash-table starts to ignore isLogicallyDeleted items

The collections eraser needs triggering so that items are actually purged