drain inFlight packets in read/write methods
drain inFlight packets in read/write methods

Fixes issue #167

File read/write client methods were breaking out of the packet handling
loops with packets still 'in flight' (packets sent without reply from
server) when errors occured. This caused problems after the fact with
the returning packets interfearing with later calls.

This change makes sure all the in flight packets are processed (even if
just discarded) before breaking out of the loop and returning.

pkg/sftp,fuchsia-mirror/third_party-github.com-pkg-sftp,ScriptRock/sftp,fuchsia-mirror/third_party-pkg-sftp