`plugins.GetAll()` was not locking the plugin map when checking if a
plugin exists, this can cause a race and potentially a panic if another
goroutine is trying to load a plugin into the map at the same time.

Also fixes a race during activation where a plugin inserts itself into
the plugin map but does not check if something else is already there.
This is already checked before trying to activate the plugin, however
the map lock is not held for this entire period, so other plugins may be
loaded during this time.
To fix, before inserting the plugin into the map, check if one with the
same name already exists and use that instead.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>