Be gracious in deprecation of method signatures related to threadlocal removal.

Certain OpenStack projects that use pecan have overridden private methods of
`pecan.rest.RestController` for custom behavior, and these method signatures
are changing in the next release of pecan to support optional threadlocals.  As
good citizens, let's give these projects a deprecation period to get their
implementations back in sync with upstream pecan.

Change-Id: I02823c7fb79488aea0dd8ad3266aa7f0024bc4a0
Merge "Be gracious in deprecation of method signatures related to threadlocal removal."