lib: fix init. use of nb_context to be by value not by reference

lib: fix init. use of nb_context to be by value not by reference

Pass context argument by value on initialization to be clear that the
value is used/saved but not a pointer to the value. Previously the
northbound code was incorrectly holding a pointer to stack allocated
context structs.

However, the structure definition also had some musings (ifdef'd out
code) and a comment that might be taken to imply that user data could
follow the structure and thus be maintained by the code; it won't; so it
can't; so get rid of the disabled misleading code/text from the
structure definition.

The common use case worked b/c the transaction which cached the pointer
was created and freed inside a single function
call (`nb_condidate_commit`) that executed below the stack allocation.

All other use cases (grpc, confd, sysrepo, and -- coming soon -- mgmtd)
were bugs.

Signed-off-by: Christian Hopps <chopps@labn.net>