For antrea-io#2092

Due to the message size and the implementation of Service in AntreaProxy,
the maximum number of Endpoints that AntreaProxy can support now is 800.
If the the number of Endpoints in given Service exceed 800, the extra
Endpoints will be dropped.

In AntreaProxy, OVS group is the key part of Service implementation. For
now, Antrea is using Openflow 1.3 to communicate with OVS. In previous
design, every bucket of a OVS group has five actions. Two actions for loading
Endpoint IP and port to registers and resubmit action must be reserved.The
other two actions for loading values to register can be moved to flows (in
current patch, they are moved to table 41), and then one message can hold
more bucket items. As a result, the maximum Endpoint has changed from 511
to 800. Unfortunately, to ensure AntreaProxy running correctly, the extra
Endpoints will be dropped.

- Rewrite the code of how to cut Endpoints when Endpoints is over maxEndpoints
- Replace svcInfo.String() with svcPortName.String()