Fix block writer serialized runner rejected execution

[dbw9580]

What changes are proposed in this pull request?

Mirroring #11310 for write handlers.

This PR uses a dedicated thread pool for block writer's serialized executor, mirroring what we do with block readers. The important change is using CallerRunsPolicy instead of the default AbortPolicy, which will prevent RejectedExecutionException in case of an excessive surge of RPCs.

Why are the changes needed?

Fix #16528.

Does this PR introduce any user facing changes?

No.