The backchannel should be associated with a session, it isn't really
global to the client.
We do, however, want a pointer global to the client which tracks which
session we're currently using for client-based callbacks.
This is a first step in that direction; for now, just reshuffling of
code with no significant change in behavior.
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
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