Multi-version client 2.0

This is a proposal to make obtaining a compatible libfdb_c easier for clients.

- [x] Add the concept of stable endpoints to flow transport
- [ ] Add a symbol to the c api to expose the version of the cluster
- [ ] Only run the compatible client for each database in the multiversion client
- [ ] Add support for downloading libfdb_c from the server over a stable endpoint. (TODO: How should this look in the c api? Should this happen transparently? Should there be a new symbol that downloads libfdb_c to a particular path)
- [ ] Support adding a client to the multiversion client while it's already running (if that's not already supported)
- [ ] Add the ability to upgrade a connection's checksum to the most recent protocol supported by both peers

@sfc-gh-rchen