As it stands, we allow plugin load requests to race. Not only does this
create a situation in which we may load and then immediately throw away
a plugin (potentially leaking its process), it also creates the
possibility of races when reading from/writing to the various plugin
caches. These changes serialize all plugin loads and cache accesses by
running all accesses for a particular host in a single goroutine.
Fixes#1020.
dc36b9569a