Add eplugin:wait_for_init/0 to avoid startup race loading plugins

Currently if you are planning to use an eplugin hook during your
application startup phase, there is a race between the result of
the "provide" cast to eplugin_srv, and the continuation of the
startup process. If the cast does not get handled before the
startup continues, your hook may run with an empty ets table, and
so no plugins are found.

To avoid paying the price of waiting for everyone, when only some
situations need to wait, this adds eplugin:wait_for_init/0, which
will only return once 'eplugin' is provided in eplugin_srv (ie, all
plugins whose dependencies can be met initially at startup are
loaded).