Correct inability to extend payload for rts msg

This change allows the application to increase the payload size
of an existing message buffer. This is needed when and application
must use the return to sender (rts) function to send a payload which
is larger than the payload in the received message.

Changes include a new RMR function: rmr_realloc_payload() and
changes to the python wrapper which will resize the payload if
the python application attempts to populate a message buffer which
does not have sufficent size.

Signed-off-by: E. Scott Daniels <daniels@research.att.com>
Change-Id: Ia55c0ff1d5ab3ce73292c00264df7bc16e229d3a