SPV syncer expects cfilters to always be available for announced blocks

[jrick]

This is unfortunately not the case, and causes peers to be dropped for stalling when cfilters are queried for announced sidechain blocks. Upon receiving the wallet's getcfilterv2 message, dcrd attempts to fetch the filters directly from the database, but these filters are only recorded when a block is attached to the best chain and lookups will fail if the block is is part of a sidechain.