Between encoding and decoding of a sync point, the node might have been
restarted and resharded with increased shard count. During resharding,
existing hints segments might have been moved to new shards. Because of
that, we need to make sure that we wait for foreign segments to be
replayed on the new shards too.
This commit modifies the sync point decoding logic so that it places a
zero replay position for new shards. Additionally, a (incorrect) shard
count check is removed from `storage_proxy::wait_for_hint_sync_point`
because now the shard count in decoded sync point is guaranteed to be
not less than the node's current shard count.
Adds a sync_point structure. A sync point is a (possibly incomplete)
mapping from hint queues to a replay position in it. Users will be able
to create sync points consisting of the last written positions of some
hint queues, so then they can wait until hint replay in all of the
queues reach that point.
The sync point supports serialization - first it is serialized with the
help of IDL to a binary form, and then converted to a hexadecimal
string. Deserialization is also possible.
