Core: don't load bloom filters by default

This change just changes the default for index.codec.bloom.load to
false: with recent performance improvements to ID lookup, such as
#6298, bloom filters don't give much of a performance gain anymore,
and they can consume non-trivial RAM when there are many tiny
documents.

For now, we still index the bloom filters, so if a given app wants
them back, it can just update the index.codec.bloom.load to true.

Closes #6959