blockchain/indexers: Add methods on CfIndex for batch retrieval.
blockchain/indexers: Add methods on CfIndex for batch retrieval.

Fetching all items in one db transaction will save time when
responding to getcfilters or getcfheaders requests.

ltcsuite/ltcd,dajohi/btcd,ppcsuite/ppcd,ltcsuite/ltcd,tb00/btcd,btcsuite/btcd,ppcsuite/ppcd,davecgh/btcd,wallclockbuilder/btcd,dajohi/btcd,wallclockbuilder/btcd,btcsuite/btcd,tb00/btcd,aakselrod/btcd,davecgh/btcd,aakselrod/btcd,dajohi/btcd