Implement writeSnapshotTarTo method, & more

This commit brings an initial high level implementation for the
generation of snapshots based off of currently derivable data in
accountsdb. It also makes a number of changes to accommodate testing,
and improve code robustness, namely a switch from byte slice paths to
directory handles in related & auxiliary code.
Also this specifically modifies loadAndVerifyAccountsFiles to take
the task indexes in order to slice into the file map data, instead of
the file name list.

Another notable change is the fixup for the `largest_root_slot` field
not actually being assigned the largest available root slot.

There are also various other improvements and refactors.