[FAB-872] Gossip multi-channel: channel

This commit adds:
1) Needed protobuf changes for multichannel support in gossip
2) A channel, an object that takes care of all multi-channel
   message types.

Signed-off-by: Yacov Manevich <[email protected]>
Change-Id: I08a9f89586d8e2562b51ea2afbc7eb0633dea360

Author: yacovm