CLEANUP: stats/cli: take the "show stat" context definition out of the appctx

CLEANUP: stats/cli: take the "show stat" context definition out of the appctx

This makes use of the generic command context allocation so that the
appctx doesn't have to declare a specific one anymore. The context is
created during parsing (both in the CLI and HTTP).

The change looks large but it's particularly mechanical. The context
initialization appears in stats.c and http_ana.c. The context is used
in stats.c and resolvers.c since "show stat resolvers" points there.
That's the reason why the definition moved to stats.h. "show info"
and "show stat" continue to share the same state definition for now.

Nothing else was modified.