Currently, we use storage_proxy/get_cas_shard ->
sharder.shard_for_reads to decide which shard to use for LWT code
execution on both replicas and the coordinator.
If the coordinator is not a replica, shard_for_reads returns 0 —
the 'default' shard. This behavior has at least two problems:
* Shard 0 may become overloaded, because all LWT coordinators that are
not replicas will be served on it.
* The zero shard does not match shard_for_reads on replicas, which
hinders the "same shard for client and server" RPC-level optimization.
To fix this, we need to know whether the current node hosts a replica
for the tablet corresponding to the given token. Currently, there is
no API we could use for this. For historical reasons,
sharder::shard_for_reads returns 0 when the node does not host the
shard, which leads to ambiguity.
This commit introduces try_get_shard_for_reads, which returns a
disengaged std::optional when the tablet is not present on
the local node.
We leave shard_for_reads method in the base sharder class, it calls
try_get_shard_for_reads and returns zero by default. We need to rename
tablet_sharder private methods shard_for_reads and shard_for_writes
so that they don't conflict with the sharder::shard_for_reads.
801bf42ea2