[jdt-core-dev] Activating registered Classpath Variable initializers

["Philippe Mulet" <Philippe_Mulet@xxxxxxx>]

Currently, all classpath variables (e.g. JRE_LIB) have their value
persisted, so as to be reused at the next session.

Some variable owners can register an initializer (through extension point),
which will be used to compute the variable value, as soon as the variable
value is required, and ONLY if no value was persisted in a previous
session.

We are getting complaints about this behavior (e.g. bug 24001), because
this prevents some registered initializers to rebind these variable
properly if something has changed in between the 2 sessions.

The proposed action is to only consider persisted values for variables, if
no initializer is found in the next session for the same variable.