Adapt legacy node request handlers to use hash_t keys internally

[gavin-norman-sociomantic]

Actually changing the on-the-wire protocol to send integers instead of strings would be a breaking change, but we can adapt the node-side request handlers to use hash_t internally. This allows storage engine implementations to be purely hash_t based, as well.

[gavin-norman-sociomantic]

This would be a breaking change, as code that uses this library (i.e. dhtnode) would break as a result of the change. (We could find a way to do this via a deprecation, instead of as a breaking change, but it would be very unfun.)

[gavin-norman-sociomantic]

Moved to v15.0.0.

[gavin-norman-sociomantic]

I think what this is talking about is the abstract methods in the node-side request classes, e.g. https://github.com/sociomantic-tsunami/dhtproto/blob/v14.x.x/src/dhtproto/node/request/Put.d#L182. The keys are currently passed around as cstrings, including at the point when they're passed into the storage engine's methods (e.g. https://github.com/sociomantic-tsunami/dhtnode/blob/master/src/dhtnode/request/PutRequest.d#L127). Moving the conversion from string to hash_t to the protocol classes, and always passing hash_t keys into the abstract methods will allow us to simplify the storage engine code.