BUG-8402: fix sequencing with read/exists requests

BUG-8402: fix sequencing with read/exists requests

When replaying successful requests, we do not issue read and exists
requests, as they have already been satisfied, but account for their
sequence numbers.

This does not work in the case where we have a remote connection,
the first request on a transaction is a read and after it is
satisfied subsequent requests are replayed to a different backend
leader.

Since the initial request is not replayed, but subsequent requests
account for it and the backend has no prior knowledge of the
transaction, it sees an initial request with sequence != 0, and
rejects all requests with an OutOfOrderRequestException.

Fix this by introducing IncrementTransactionSequenceRequest, which
the frontend enqueues as the first request instead of the initial
read/exist request -- introducing the transaction to backend.

Change-Id: Ia0f048e33d417e1fdc8d15bf319d6b8b33c2b1b1
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>