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c9294b1642
Instead, use shard_for_reads(). The justification is that:
1) In cas_shard(), we need to pick a single request coordinator.
shard_for_reads() gives that, which is equivalent to shard_of()
if there is no intra-node migration.
2) In paxos handler for prepare(), the shard we execute it on is
the shard from which we read, so shard_for_reads() is the one.
3) Updates of paxos state are separate CQL requests, and use their
own sharding.
4) Handler for learn is executing updates using calls to
storage_proxy::mutate_locally() which will use the right sharder for writes
However, the code is still not prepared for intra-node migration, and
possibly regular migration too in case of abandoned requests, because
the locking of paxos state assumes that the shard is static. That
would have to be fixed separately, e.g. by locking both shards
(shard_for_writes()) during migration, so that the set of locked
shards always intersects during migration and local serialization of
paxos state updates is achieved. I left FIXMEs for that.