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otherwise the next pull request will likely return outdated values,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.
In several places in gossip code, the entire crds table is scanned only
to filter out nodes' contact infos. Currently on mainnet, crds table is
of size ~70k, while there are only ~470 nodes. So the full table scan is
inefficient. Instead we may maintain an index of only nodes' contact
infos.
Based on run-time profiles, the majority time of new_pull_requests is
spent building bloom filters, in hashing and bit-vec ops.
This commit builds crds filters in parallel using rayon constructs. The
added benchmark shows ~5x speedup (4-core machine, 8 threads).
