Computing the bundle plugin classpath can be an expensive operation for
different reasons. In many cases these computation even yields the same
results as before especially on workspace restart without target
changes.

This now stores the last successful computed state per project and read
it back on initialization of the classpath container, scheduling a check
for actual changes in the background. Only if this check yields a
different result or the update is requested as part of a change to the
manifest or target state for example the current values are updated.

This yields faster startup times without the risk of blocking (other
than file I/O) until PDE can deliver the classpath to consumers delaying
the heavy work to a later time.