remove colony plugin_startup, moving all its code elsewhere

Previously start was called by default during plugin_startup. Now it works like in delayed start mode, that is, it waits for first block to activate the plugin. Also the signal handlers are now assigned in plugin_initialize like in other plugins. The change was necessary, because assumption that plugin_startup has to fully complete before hived starts working turned out to be false. As a result calling start during plugin_startup was not protected from background changes in indexes. Delayed start is protected because it is executed in post_apply_block.