Revert "MINOR: opentracing: change the scope of the variable 'ot.uuid' from 'sess...

Revert "MINOR: opentracing: change the scope of the variable 'ot.uuid' from 'sess' to 'txn'"

This reverts commit 560c7b874aef5922199e36a7f31466af323f489f.

The ot.uuid variable should have the 'sess' scope because it is created
when an OpenTracing filter is attached to a stream.  After that, the
stream processing is started and on that occasion the contexts for the
variables that have the range 'txn' and 'req' are initialized.  This
means that we cannot use variables with the specified scopes before that
point.

This patch must be backported in 2.5.