Update getrawmempool RPC method

[mpguerra]

Motivation

In order to enable zcashd deprecation, we need to ensure that RPC methods that are used by existing zcashd consumers are implemented in zebra to cater to their usecases.

Specification

zcashd's getrawmempool RPC method is documented here: https://zcash.github.io/rpc/getrawmempool.html

Zebra already partially implements the getrawmempool RPC method, however, it does not implement the optional verbose argument.

We need to:

1. Check whether any existing zcashd consumers use this parameter
2. If so, implement the required changes to support the relevant usecases.