base64 encode correlation_id in rpc requests

This prevents the rpc client from sending an invalid correlation_id
when the counter reaches 128.  The 0.9.1 AMQP spec defines the
property as a "short string" which requires it to be valid UTF-8.

The encoded identifier is also used as the dictionary key for the
continunation to avoid decoding the correlation id when handling the
response.
Cosmetic
Merge bug 25489