MINOR: stktable: Use an enum to type a sticky session in the updates tree

MINOR: stktable: Use an enum to type a sticky session in the updates tree

Instead of using a boolean to know if an entry in the updates tree is local
or not, an enum is used. This change will be mandatory when updates tree
will be replaced by a list to be able to add markers owned by each peer.

So now a sticky sessin has no type (STKSESS_UPDT_NONE) if it is not in the
updates tree. STKSESS_UPDT_LOCAL is used for local entries and
STKSESS_UPDT_REMOTE for remote ones. STKSESS_UPDT_MARKER is not used for
now.