BUG-8402: correctly propagate read-only bit

During replay we substitute read requests with an IncrementSequence
request, but that does not indicate whether the transaction state
should be read-only.

This leads to transaction chains allocating a full-blown transaction
instead of a snapshot, hence follow-up transactions fail to allocate,
leading to OutOfOrderRequestException.

Fix this by making IncrementTransactionSequenceRequest a subclass
of AbstractReadTransactionRequest so it carries isSnapshotOnly().

Change-Id: Ifdb6214478aa7548d3bc1f06b532e06c93b3dd0b
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>