BlameCacheImpl: initialize weigher in newBuilder()

The weigher implementation should not change. Since we can now merge a
partially-initialized builder with the options from a config, we can
push the weigher setup into newBuilder(). (The loader still may only
be initialized from the constructor since it is passed to build().)

Change-Id: I9c15e4876dd8fcd517cae513099aa239dc82112c