perf(engine): Make consensus WAL appends non-blocking

[Raneet10]

Closes: #1435

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WAL appends in the consensus routine were blocking since each Effect::WalAppend issued a synchronous ractor::call! to the WAL actor. Since WalMsg::Append is an asynchronous write by nature (and only WalMsg::Flush must be synchronous), this blocking was unnecessary overhead. Hence, the WAL appends are now queued and awaited only before flush operation.

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