Make runTLS{Client,Server}StartTLS general

[abbradar]

This makes it possible to use general runTLSClientStartTLS without losing monadic state, and also generalizes runTLSServerStartTLS. Consists of several parts:

1. Make top function in ApplicationStartTLS return a -- this allows us to return and re-introduce wrapped monadic state with liftBaseOp.
2. Also make it parametrized by a monad. This is to not lose monadic state from the nested startTLS function. Another possible solution would be to use impredicative types to allow returning value from it (replace (AppData -> IO ()) -> IO () with forall a. (AppData -> IO a) -> IO a and use ordinary liftBaseOp). Unfortunately, GHC doesn't work well with this.
3. Finally, generalize both runTLS*StartTLS functions.

Also this allows one to return value from runTLSClientStartTLS (not strictly needed for this patch) and drops an extra constraint from runTLSClient.

I'm not sure if this needs to be done some other way w.r.t. backwards compatibility.

EDIT: Also notice that (1) is also not strictly required as we already have (2) for passing state. If needed, I can split this into a different PR.

[snoyberg]

Changing the existing type synonym breaks backwards compatibility, so I'd rather just define a new type synonym. Other than that, changes look backwards compatible.

[abbradar]

Fixed!

[snoyberg]

Thanks!