This fixes a portion of the bug #442, coordinator crash on COPY TO.
Current code did not do any flow control so the coordinator continues
to send row data to the datanode even though datanode is very busy and
cannot receive the data, thus exceeding the buffer size limit (1GByte).

This patch monitors how much data are sent by examining send() return
value.   If outstanding data exceeds a limit, it retries to send
until the size of outstanding data becomes small enough.

This fix deals with COPY TO.   Fix to COPY FROM will be done as
a separate commit.

COPY TO crash report was made by Mason Sharp.