io.EOF is never expected during valid usage. In addition, database/sql
uses io.EOF as a sentinal value that all rows from a query have been
received.
See https://github.com/jackc/pgx/issues/662.
When an message is received and a timeout occurs after reading the
header but before reading the entire body the connection state could
be corrupted due to the header being consumed. The next read would
consider the body of the previous message as the header for the next.
fixes#348
