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.
Fail predictably when connecting to a refusing server
Reader changes
Also blow up if we get a completely malformed header
Merge bug 25502
stable to default
Use defines. Also, 4 =/= 8 (the PDF version of the spec LIES).
Unused variables.
stable to default
Merge 3.1.0 into stable.
Added tag rabbitmq_v3_1_1 for changeset 6b38bfb06773
document auth_mechanism query param in amqp URIs
Use the negotiated frame-max when opening channels
Cosmetic
Merge bug 25489
Cosmetic
Strip extraneous comment
Merge bug 25521
unbreak edoc
fix channel 0 exit handling