FLUME-2404. Make ScribeSource read buffer and max frame size configurable

Scribe default Thrift service maxReadBufferBytes and frame size varies
across Thrift versions. In some cases, these values are set to INT_MAX,
in other cases this is set to 16MB. To avoid OOM in certain cases and
incompatibilities in other cases, set the default to 16MB and also make
the parameters configurable.

(chenshangan and Marimuthu Ponnambalam via Mike Percy)