Pair requests with replies using sequence numbers

Pair requests with replies using sequence numbers

Requests are given sequence numbers (cycling over a finite set of 2^16
integers) that are stored into the lower 16bits of the context.
1bit is also allocated for isMultipart boolean flag and the remaining
15bits are used to store the channel ID. The sequence numbers allow
to reliably pair replies with requests, even in scenarious with timeouted
requests or ignored (unread) replies.

Sequencing is not used with asynchronous messaging as it is implemented
by methods of the Channel structure, i.e. above ReqChan and ReplyChan
channels.

Change-Id: I7ca0e8489c7ffcc388c3cfef6d05c02f9500931c
Signed-off-by: Milan Lenco <milan.lenco@pantheon.tech>