serverdb: missing support for multiple operations on same row

[flavio-fernandes]

When a transaction has more than one operation that affects the same row of a table,
it should honor the intermediary state of the row, so it does not get lost in the process
of committing the transaction.

Please refer to this PR as a reference test that better explain the issue -- and fixes the issue. ;)
#233

[flavio-fernandes]

~/dev/libovsdb.git issue232 vagrant@devmaster
❯ make test

=== RUN   TestMultipleOps
    transact_test.go:222:
                Error Trace:    transact_test.go:222
                Error:          Not equal:
                                expected: ovsdb.TableUpdates{"Bridge":ovsdb.TableUpdate{"a7fdad81-9187-4a3c-a95e-585f9ebe9714":(*ovsdb.RowUpdate)(0xc00066b4f0)}}
                                actual  : ovsdb.TableUpdates{"Bridge":ovsdb.TableUpdate{"a7fdad81-9187-4a3c-a95e-585f9ebe9714":(*ovsdb.RowUpdate)(0xc00066b140)}}

                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -9,3 +9,3 @@
                                     (string) (len=5) "ports": (ovsdb.OvsSet) {
                                -     GoSet: ([]interface {}) (len=4) {
                                +     GoSet: ([]interface {}) (len=3) {
                                       (ovsdb.UUID) {
                                @@ -15,5 +15,2 @@
                                        GoUUID: (string) (len=6) "port10"
                                -      },
                                -      (ovsdb.UUID) {
                                -       GoUUID: (string) (len=5) "portA"
                                       },
                Test:           TestMultipleOps
--- FAIL: TestMultipleOps (0.01s)