[3.5] server: don't panic in readonly serializable txn

[lavacat]

trafficstars

Problem: We pass grpc context down to applier in readonly serializable txn. This context can be cancelled for example due to timeout. This will trigger panic inside applyTxn

Solution: Only panic for transactions with write operations

fixes https://github.com/etcd-io/etcd/issues/14110 backported from main https://github.com/etcd-io/etcd/pull/14149

Signed-off-by: Bogdan Kanivets [email protected]

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[lavacat]

PR isn't merged yet, but even if solution changes, I think test is relevant

[ahrtr]

Please update this PR per https://github.com/etcd-io/etcd/pull/14149

[lavacat]

Sorry about delay. Will update soon.

[lavacat]

@ahrtr @ptabor FYI, this is ready to merge

[ahrtr]

This is just a backport PR, so merging...

Please add a changelog item if you haven't done it. @lavacat