Add support for Delete of entries.

[robshakir]

 * (M) fluent/fluent.go
 * (M) fluent/fluent_test.go
   - Add Status() to retrieve client status in tests - this allows for send and
     receive errors to be checked.
   - Add DeleteEntry() function to submit a set of entries to be deleted by the
     server, and corresponding ReplaceEntry() function.
 * (M) rib/rib.go
 * (M) rib/rib_test.go
   - Add top-level DeleteEntry function that can take an operation from the caller.
   - Fix DeleteIPv4 to not check content as per design discussion.
   - Add DeleteNextHopGroup and DeleteNextHop.
   - Test coverage for individual DeleteXXX and the overall DeleteEntry.
   - Improve test coverage by injecting check functions that inject errors and
     programming failures.
 * (M) server/server.go
 * (M) server/server_test.go
   - Add support for delete and replace in doModify by generalising addEntry.
     This ensures that we do not have multiple places in the implementation
     that need to be aware of each operation.

[robshakir]

Thanks for the review!