THRIFT-388. Create a "ONEWAY" message type that is an alias for "CALL"

THRIFT-388. Create a "ONEWAY" message type that is an alias for "CALL"

Pave the way for a new message type for oneway function calls.
For now, just define the constant in all languages and make
server implementations treat it the same way as a normal call.
Only C++ and Erlang currently check the message type (on the
server side).

There is a little bit of redundancy in the Erlang code, but
the alternative is a bit gross, and this split-up will be
necessary eventually when we start handling one-way calls
differently.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@761389 13f79535-47bb-0310-9956-ffa450edef68