IMPALA-1565: Python sasl client transport perf issue

The shell uses the thrift sasl client (thrift_sasl.py) for communication with
impalads when using ldap and/or kerberos. The sasl client currently makes calls
into sasl for every buffer repeatedly even when it is not necessary, resulting
in a significant performance degradation when using the impala-shell w/ kerberos
and/or ldap.

thrift_sasl.py was forked from hue at some point, and this change updates the
code to reflect the updated code as of hue commit a9898b4e815b3ec9918c5db65e0d9bd1d0ecdde0
which only calls into sasl when frames are encoded.

Change-Id: Ic2194d51c2c4470d48c617c054ba8e90053052f9
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5482
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins