datapath-windows: pid-instance hash table data structure.

This patch introduces data structure for holding instances hashed by pid.

Signed-off-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
cked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>