Simplify lazy jmethodID cache in InstanceKlass

InstanceKlass has a jmethodID cache that lazily grows after redefinition upon request for a new jmethodID. This lazy growth is because for obsolete methods, the method idnum is incremented.
This lazy growth requires that jmethodIDs may have to be deleted if there's a race in creating them.
Since redefinition is at a safepoint, we can just extend the jmethodID cache then since we know the count.