channel: use real jsonrpc & cleanup package

[yhabteab]

This PR is a follow-up to the previous PR of mine, which introduced the jsonrpc package in Icinga Go Library (see the requires section below). This PR now makes use of the jsonrpc package and cleans up the channel package a bit. It was a bit messy before because we didn't change anything in that package since its initial implementation. There is no behavior change in this PR apart from the following two changes:

1. Since the new jsonrpc package allows for a bidirectional communication, Icinga Notifications acts now as a JSON-RPC client and server at the same time. Meaning, for the former case, it will continue to send JSON-RPC requests to the channel plugins, and for the latter case, it will now also be able to receive JSON-RPC requests from the channel plugins. Currently, it only implements a Log method, which allows channel plugins to send log messages with fine-grained log level and ontexts to Icinga Notifications. This is useful for debugging purposes, as it allows channel plugins to log messages directly to the Icinga Notifications log as opposed to the currently used stderr mechanism to report errors.
2. The second behavior change is that Icinga Notifications will not restart the channel plugins with each config change anymore. Instead, it will just call the SetConfig method on the channel plugins, which allows them to replace their configs without restarting. The only time a channel plugin will be restarted is when it crashes or when the channel.type changes in the config.

Requires

• notifications: replace rpc with real jsonrpc package icinga-go-library#226