Implement POST /lsp/jitchannel from LSP spec

[JssDWt]

The proposed JIT channel spec exposes an endpoint GET /lsp/jitchannel, which reserves a SCID for the client, which it can use in a routing hint to have a JIT channel opened by the LSP.

• if version is not 1: error
• if the promise is not valid: error
• if valid_until expired: error
• if payment_size falls out of range: error
• if channel_size falls out of range: error
• if channel size * 99% > payment size: error
• reserve a unique SCID for this client until the expiration period

Note that the SCID has to be persisted along with the client request parameters.

Depends on promise generation defined in #70
Depends on SCID generation defined in #72

[JssDWt]

Closing in favor of #108