Change package names for rpc sync and related handler classes

Suggestion was made to move RPC classes from `com.swirlds.platform.gossip.shadowgraph` to `com.swirlds.platform.gossip.sync.protocol` or similar, but at the same time, RpcPeerHandler is not in any way related to protocol concept.

Better structure of the packages is needed after required refactorings are done with the control and data flow.

No files or tests are named. Start by locating the RPC classes in com.swirlds.platform.gossip.shadowgraph and RpcPeerHandler, then review the control and data flow refactorings referenced in the issue. Done means the package structure has a decided separation for RPC and handler classes, with all package declarations and references updated consistently.

Written by the indexing model from the issue text.