Fix a deadlock in openrsync when big files are synced using the hash

Fix a deadlock in openrsync when big files are synced using the hash
algorithm. Make sure the sender does not run ahead of itself and end
stalling in a read from network that never shows up. Instead ensure
that all queued data is pushed out before accepting new data.

Problem found by and fix developed with Kyle Evans (kevans freebsd.org)
OK tb@ deraadt@