Fix various things in Solr startup

This was originally written to address #134.  Mainly, it was to change `wait_for_solr` from throwing, causing the crash of the `yz_solr_proc:init` function and thus causing the whole node to stop.  It turns out that even without the throw, anything but `{ok, State}` will cause the supervisor tree to shutdown completely.  It's worse than that, the init code has a default of 5s to start according to @beerriot, so the `wait_for_solr` timeout isn't actually even honored.

The new plan is to cause the Riak node to have a full shutdown if the Solr process cannot stay up.  @beerriot will be making a PR to move the start out of the `init` function.  Until then various fixes have been made around the Solr startup process.  The main changes were to avoid throwing/exiting inside the process, instead use the `stop` directive for `gen_server`, and to move the solr proc directly under `yz_sup` in order to make sure max restart frequency can actually be reached and cause the Riak node to shutdown.