This is in preparation of moving mempool to its own subpackage.  No
functional change.

This creates a skeleton mining package that simply contains a few of the
definitions used by the mining and mempool code.

This is a step towards decoupling the mining code from the internals of
btcd and ultimately will house all of the code related to creating block
templates and CPU mining.

The main reason a skeleton package is being created before the full
blown package is ready is to avoid blocking mempool separation which
relies on these type definitions.

DisableRelayTx sets the DisableRelayTx value in the version
message which informs the remote peer on whether to send
inv messages for transactions.

Contains the following upstream commits:
- 83bcfea
- 2f6aeac
  - Cherry-picked because it fixes an issue originally introduced by the
    commit being merged

In addition to the normal required changes for syncing, the following
changes have been made in order to facilitate integration into Decred:

- Configure the NewestSha function to the new mempool config as a
  closure over the block manager state instead of using the database
  func directly
- Add a new NextStakeDifficulty callback to the new mempool config for
  obtaining the next stake difficulty from the block manager and
  configure it as a closure over the block manager state.

Upstream commit ce981f4.

Upstream commit 2a7f41c.