This serves as the final release of version 1 of the mempool module.
All future releases will be moving to version 2 of the module.

Consequently, it bumps the module version as follows:

- github.com/decred/dcrd/mempool@v1.2.0

It also removes the mempool override in the root module and updates it so building the
software will still produce binaries based on the v1 module until the v2
module is fully released.

This removes the deprecated MinHighPriority constant as part of moving
to mempool/v2.

Note that since this constitutes a major break to the API, the mepool
module version will need to be bumped to version 2.

Currently, there is a tight coupling between the rpcserver and the
mempool by way of the RawMempoolVerbose function returning a
dcrjson.GetRawMempoolVerboseResult.

In order to remove the tight coupling, this introduces a new type and
function to the mempool module named VerboseTxDesc and VerboseTxDescs,
respectively, which provide all of the details necessary for the
rpcserver to be able to properly generate the dcrjson response
directly.

Also, it removes the old RawMempoolVerbose function and related
dependency imports on dcrjson since it is no longer required as a part
of moving to mempool/v2.

Note that since this constitutes a major break to the API, the mepool
module version will need to be bumped to version 2.

This updates the docs/README.md file, module hierarchy graphviz, and
module hierarchy diagram to reflect the new module version