Fix bug with socket writing.

Fix bug with socket writing.

pool_write_flush() is responsible for writing to sockets when pgpool's
write buffer is full (this function was introduced in 3.6.6 etc). When
network write buffer in kernel is full, it does retrying but it forgot
to update the internal buffer pointer. As a result, broken data is
written to the socket. This results in variety of problems including
too large message length.