update internal modeling for plugins

[edaniszewski]

This PR just updates the plugin modeling to something more sensible for Synse v3. It strips away some of the complexities of the v2 implementation. Particularly, it:

• attaches all functionality to the plugin manager
• simplifies the relationship between manager and plugin. plugins no longer self-register on init, the manager is not class member to the plugin.
• removes the validation checks for unix plugins -- since the client will need to be initialized and used for the plugin to be initialized, a failure in the config will result in a failure to initialize
• removes loading plugins from the default socket location. this was only complicating the code in a number of ways (plugin loading, validation, config sync) for a feature that we have not used at all.

It does not update any documentation or usage, so things will break with this PR, but upcoming PRs will drastically change components that use this, so there is no point in updating those just to change them again shortly.

Related:

• should synse remove/disable registered plugins if they become unresponsive? #286