Provide a default searchRecords implementation.

Provide a default searchRecords implementation.

This is the LMDB searchRecords being promoted to backend-agnostic, as it
only depends on list() and getAllDomains() to work (and get(), obviously).

A new capability, CAP_SEARCH, is added to advertize these requirements are
fulfilled; search will return failure if they are not.

Signed-off-by: Miod Vallat <miod.vallat@powerdns.com>