[mempool] Rename overloaded PeerInfo type
[mempool] Rename overloaded PeerInfo type

`PeerInfo` is confusing, as it's a map, and the info is SyncState, which
contridicts the similar naming in StateSync.  This hopefully makes it a
little less confusing, with overloaded peer terms.

aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra