## Why this should be merged
Allow `ava-labs/coreth` to use arbitrary `triedb` database
implementations.
## How this works
Introduces `HashBackend` and `PathBackend` interfaces that
`triedb.Database` type-asserts to instead of `hashdb.Database` and
`pathdb.Backend` respectively. Other interfaces are introduced to avoid
having to modify `{hash,path}db.Database.Reader()` return values.
The explicit `DBOverride` field means that we don't have to modify any
of the `triedb` constructor beyond adding a single line immediately
before the return. This leaves all modifications of original files
entirely mechanistic. This is, however, at the expense of compile-time
guarantees of the overriding database being either a `HashBackend` or a
`PathBackend`.
## How this was tested
Unit test demonstrating override + plumbing.
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Co-authored-by: Darioush Jalali <darioush.jalali@avalabs.org>
