Hot config reloads

[jleni]

It would nice to have something similar to tmkms refresh to reload changes in the configuration file without having to restart the complete KMS.

Yeah, we should definitely support hot config reloads. Feel free to open an issue.

Originally posted by @tarcieri in #177 (comment)

[liangping]

how about tmkms reload

[tarcieri]

Abscissa 0.1 (unreleased) now contains the prerequisites for hot config reloads, including a signal-hook based signal handler which processes all signals by obtaining an exclusive mutable reference to the application state.

KMS doesn't use it yet: I'm trying to keep the initial Abscissa upgrade as light touch as possible. But after KMS v0.6 is released, we should be able to switch to it and handle config reloads via e.g. SIGHUP.

After that, we could potentially add friendlier ways to send that SIGHUP like @liangping's suggested tmkms reload.