move template class member to correct TU for explicit instantiation

[graydon]

During recent refactoring a single method of the template <class BucketT> class SearchableBucketListSnapshotBase -- the method SearchableBucketListSnapshotBase<BucketT>::loadKeysInternal -- got left behind in SearchableBucketList.cpp while the rest of the methods got moved to BucketListSnapshotBase.cpp.

Unfortunately all explicit template instantiations of SearchableBucketListSnapshotBase occur in BucketListSnapshotBase.cpp and they only cause instantiation of methods defined in the same TU. So the method didn't get instantiated. At least on g++.

This was covered up in CI because enabling extrachecks winds up triggering instantiations anyways. But if you turn off extrachecks, master doesn't currently compile on g++.

This fixes the bug by moving the method to the right TU.

[SirTyson]

LGTM, thanks for tracking this down! Apologies for introducing some templated complexity with this one...

[graydon]

@SirTyson I appreciate your ongoing efforts to keep well-factored what has turned into a real house of mirrors! At some point when the dust has settled it might be useful to draw an ascii-art diagram of all the classes in bucket/ just to help orient future readers :)