ovsdb: Add/use partial set updates.

ovsdb: Add/use partial set updates.

This patchset mimics the changes introduced in

  f199df26 (ovsdb-idl: Add partial map updates functionality.)
  010fe7ae (ovsdb-idlc.in: Autogenerate partial map updates functions.)
  7251075c (tests: Add test for partial map updates.)
  b1048e6a (ovsdb-idl: Fix issues detected in Partial Map Update feature)

but for columns that store sets of values rather than key-value
pairs.  These columns will now be able to use the OVSDB mutate
operation to transmit deltas on the wire rather than use
verify/update and transmit wait/update operations on the wire.

Side effect of modifying the comments in the partial map update
tests.

Signed-off-by: Ryan Moats <rmoats@us.ibm.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>