For all code paths (gossip push, pull, purge, etc) that remove or
override a crds value, it is necessary to record hash of values purged
from crds table, in order to exclude them from subsequent pull-requests;
otherwise the next pull request will likely return outdated values,
wasting bandwidth:
https://github.com/solana-labs/solana/blob/ed51cde37/core/src/crds_gossip_pull.rs#L486-L491
Currently this is done all over the place in multiple modules, and this
has caused bugs in the past where purged values were not recorded.
This commit encapsulated this bookkeeping into crds module, so that any
code path which removes or overrides a crds value, also records the hash
of purged value in-place.
9d112cf41f