[roll] Roll fuchsia [block_server][c] Track request state

The server keeps track of active request state so the C++ side doesn't
need to be trusted to do so.  The C++ side now use request IDs which
are indices into the internal request state map, and respond to requests
by ID.

This was necessary because C clients which asynchronously respond to
messages don't work with the current API, where BlockServer::OnRequests
passes in a const Session& (expecting a synchronous call to
Session::SendReply before OnRequests returns).  The client can't just
store the Session as a pointer because this is a temporary wrapper
object.  One option was to create a ref type (e.g. SessionRef) and pass
that in, but there is a greater change of use-after-free type issues
with that, versus the approach taken here.

This has several other advantages:
- Internally we don't need to use high-bits in request IDs to decide
  whether to discard the VMO on request completion; the internal request
  state keeps a strong reference to the VMO if needed.
- We were able to get rid of the hack to create a Session that didn't
  run its destructor in block_server.cc.
- The C API becomes a bit simpler.

Original-Bug: 410926221
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1255385
Original-Revision: b1916a0641b39a5597d957c178a79bbc266b562a
GitOrigin-RevId: 39b111e851c204b48ce28883763255d394191466
Change-Id: Ic7d0c50a24a99594f5369a441f47a27b1ebb8876