seq: fix problem with synchronous pvPut/pvGet

The problem was that that once a synchronous request times out, the
variable would never recover from the timeout state. This was because
the semaphores would not be signalled on timeout. However, in order
to correctly discard events (i.e. callbacks from CA) that belong to
timed out requests, we have to remember them. This is what the getReq
and putReq arrays in struct state_set are for.

The fix assumes that CA will eventually call the callback for each
accepted request, even if the channel disconnects. If the assumption is
wrong then we have a memory leak.