This removes the dependency that the UtxoCacher interface and UtxoCache
implementation currently have on the database.Tx interface.  This is
necessary since the UtxoCache will move away from the current database
interface in favor of an interface that is better suited for the UTXO
database in a subsequent commit.

This adds a UtxoBackend interface and LevelDbUtxoBackend type, which
implements that interface.  This UtxoBackend interface will allow
different backend implementations to be used by the UtxoCache rather
than interacting directly with the database layer as it does currently.

In subsequent commits, interface methods and the corresponding
implementation will be added to UtxoBackend one at a time in order to
ease the review process as things move around.

This exports the UtxoSetState type since it will be used in the
UtxoBackend interface in a subsequent commit.

This adds the FetchEntry method to the UtxoBackend interface and
LevelDbUtxoBackend implementation.

A summary of the changes is as follows:
- Add FetchEntry method to UtxoBackend interface
- Add FetchEntry method to LevelDbUtxoBackend
- Move dbFetchUtxoEntry to LevelDbUtxoBackend
- Add backend tests for FetchEntry
- Update UtxoCache to fetch entries from the backend rather than from
  the database directly

This adds the PutUtxos method to the UtxoBackend interface and
LevelDbUtxoBackend implementation.

A summary of the changes is as follows:
- Add PutUtxos method to UtxoBackend interface, which atomically updates
  the UTXO set and state
- Add PutUtxos method to LevelDbUtxoBackend
- Move dbPutUtxoEntry to LevelDbUtxoBackend
- Add backend tests for PutUtxos
- Update UtxoCache to use PutUtxos rather than updating the database
  directly

This adds the FetchState method to the UtxoBackend interface and
LevelDbUtxoBackend implementation.

A summary of the changes is as follows:
- Add FetchState method to UtxoBackend interface
- Add FetchState method to LevelDbUtxoBackend
- Add backend tests for FetchState
- Update UtxoCache to use FetchState rather than querying the database
  directly
- Remove dbPutUtxoSetState and dbFetchUtxoSetState since they are no
  longer used
- Add FetchBackendState to the UtxoCache so that the state can be
  fetched through the cache

This adds the FetchStats method to the UtxoBackend interface and
LevelDbUtxoBackend implementation.

A summary of the changes is as follows:
- Add FetchStats method to UtxoBackend interface
- Add FetchStats method to LevelDbUtxoBackend
- Move dbFetchUtxoStats to LevelDbUtxoBackend
- Add FetchStats to the UtxoCache so that the stats can be fetched
  through the cache

This adds the FetchInfo, InitInfo, and PutInfo methods to the
UtxoBackend interface and LevelDbUtxoBackend implementation.

A summary of the changes is as follows:
- Add FetchInfo, InitInfo, and PutInfo methods to UtxoBackend interface
- Add FetchInfo, InitInfo, and PutInfo methods to LevelDbUtxoBackend
- Move dbPutUtxoDatabaseInfo, dbFetchUtxoDatabaseInfo, and associated
  variables to LevelDbUtxoBackend
- Move initUtxoDbInfo, createUtxoDbInfo, and associated variables to
  LevelDbUtxoBackend
- Rename and export utxoDatabaseInfo to UtxoBackendInfo
- Add backend tests for putting and fetching the UTXO backend info
- Add utxoBackend to BlockChain Config since BlockChain handles
  initializing the UTXO backend
- Remove utxoDbInfo from BlockChain and instead access the UTXO backend
  info through the UTXO backend

This moves LoadUtxoDB and the associated helper functions and variables
from utxodb.go to utxobackend.go since LoadUtxoDB is specific to the
LevelDbUtxoBackend implementation of the UtxoBackend interface.

This adds the Upgrade method to the UtxoBackend interface and
LevelDbUtxoBackend implementation.

A summary of the changes is as follows:
- Add Upgrade method to UtxoBackend interface
- Add Upgrade method to LevelDbUtxoBackend
- Remove initUtxoState and instead handle upgrading through the
  UtxoCache Initialize method
- Pass the UTXO backend into upgradeUtxoDb rather than the entire
  BlockChain instance

This removes the UTXO db from the BlockChain and UtxoCache types.
BlockChain now only accesses the UTXO db through the UtxoCache and the
UtxoCache now only accesses the UTXO db through the UtxoBackend.

This exports the ViewFilteredSet type since it is used in the UtxoCacher
interface.