Cleaned up client start process.

A few things to note:
- The client will now save worlds correctly as its world was never set
  before. This needs to be fixed to use the SpoutClientWorld container.
- Depending on the plugin, the client will failout with sync chunk
  generation errors. @raphfrk needs to harden the check some more. Since
  Vanilla is a heavy plugin (heavy generators), it never gets this error.
Signed-off-by: Chris Sanders <zidane@outlook.com>