Initial implementation of new RPC loop

NEW FEATURE

Description of Feature

This MR implements the new RPC loop, along with a few RPCs, for the host. The RPCs implemented are: EnterLoop, ExitLoop, Settings, RecentRevision, and Download. Both the renter and host side of the protocol are implemented for these RPCs. Additional RPCs will be implemented in later MRs.

For now, I am resisting the urge to abstract this code via RPC request/response helper functions. I expect that such an abstraction will be both possible and desirable, but I am holding off until we implement a few more RPCs, because they may have additional requirements that affect the final abstraction. For example, it seems like we will want some mechanism for the host to cache some metadata within a RPC loop session (e.g. a storageObligation that is being updated on each loop iteration), but it's not clear how best to achieve this.

Testing Added

Since both the renter and host are involved, integration tests have been added to the siatest package. No new tests have been added to the host or proto packages.

Modules and Packages Impacted

This MR affects the host, proto, and modules packages. host gained support for the new RPC loop and various RPCs; proto gained a Session type that can request these RPCs; and modules gained the various IDs, request types, and response types of the new RPCs. Subsequent MRs will add new RPCs and make use of the Session type in the contractor and renter packages.