client: account activity monitor

[aubrika]

We need a utility that can monitor the activity of one or more accounts, given only a full viewing key for each.

The input is a JSON file containing FullViewingKeys, and the tool needs to be able to check that, for each account of each FVK, the current UM-equivalent balance of the account is greater than or equal to the genesis balance.

This will require:

• one-off scanning of the compact block with each FVK in order to compute the genesis allocations as they were in the genesis file
• building a directory tree that can be pcli/pclientd home directories that have view databases in them and have the current state

In the (near) future the tool will also need to handle key rotations (transfer of balances from one account to another), so this work should be done to accommodate the work done to that end in #4831.