There is a sharding edge case where more CleaveContext are generated and
stored in the sharding container DB. If this number get's high enough,
like in the linked bug. If enough CleaveContects build up in the DB then
this can lead to the 503's when attempting to list the container due to
all the `X-Container-Sysmeta-Shard-Context-*` headers.
This patch resolves this by tracking the a CleaveContext's last
modified. And during the sharding audit, any context's that hasn't been
touched after reclaim_age are deleted.
This plus the skip empty ranges patches should improve these handoff
shards.
Change-Id: I1e502c328be16fca5f1cca2186b27a0545fecc16
Closes-Bug: #1843313
81a41da542