watchtower: create a new cmd directory to enable a standalone watch tower #3290
Description
As implemented today, a watch tower can only run in a "monolith" mode beside lnd. It should be possible to run a binary that only runs a watch tower, to allow for more flexible deployment configurations. Most of the scaffolding is already in place for towers, since they're able to run as a part of lnd. The remaining work is to "daemonize" the existing scaffolding such that the tower can run on its own. This stand-lone tower binary (towerd?) should come equipped with the current watch tower sub-server, and should also be able to interact with using the lncli tower command.