Our codec implementation originally just parsed single messages out of
the stream buffer. However, if a query returns a bunch of rows, we're
spending a ton of time shipping those individual messages from the
connection back to the Query stream. Instead, collect blocks of unparsed
messages that are as large as possible and send those back.
This cuts the processing time of the following query in half, from ~10
seconds to ~5:
`SELECT s.n, 'name' || s.n FROM generate_series(0, 9999999) AS s(n)`
At this point, almost all of the remainder of the time is spent parsing
the rows.
cc #450
