Tidy up transaction tests
Simplify parserbuffer

Remove overdesigned goroutine-managed buffer type; our use case is way
more simple as we will only have one pending requestat a time, and the
parserbuffer is currently a performance bottleneck.

Replace with a simple wrapper around an io.Pipe to a bytes.Buffer.
Revert "Tidy up transaction tests"

This reverts commit 57bb0a47324bb8e57616468649721089445d3307.