seq: simplified pvPut and pvGet implementation

Instead of using per channel semaphores to check for completion, the
new implementation uses the request pointer. A NULL pointer means that
no request is pending, i.e. either the request was cancelled or it
completed sucessfully or we lost connection to the PV. The CA callback
resets the request pointer before signalling to the state set where the
request originated, that /some/ event happened. This re-uses the the
existing sync semaphore. ASYNC operations no longer use a semaphore at
all, only the SYNC variants do. However, it becomes even more important
to check whether a disconnect happened, since this is "some event", too,
i.e. it causes pending requests to be cancelled.