kvstore: access multiple tables/singletons

[nrc]

Access to tables in a transaction is via an &mut reference, which means only one table can be accessed at a time. We should be able to access multiple distinct tables and singletons at the same time.

Some ideas for implementation:

• Allow access via an immutable ref to the transaction and dynamically track the tables/singletons to ensure uniqueness
• Allow opening multiple tables at once (but this doesn't address the issue of singletons)
• Allow some way to open further tables/singletons from a table object.

[npry]

possible thought: synthesize a unique transaction struct (per kv store decl/in the macro) with fields per table and kv, then the compiler could reason normally about partial borrows on the struct fields. something shaped roughly like (~pseudocode):

pub struct TxnCtx {
    // tables:
    peers: KvTableTransactional<Peers>,
    best_paths: KvTableTransactional<BestPaths>,
    // ...

    // singletons:
    active_peers: SingletonTransactional<ActivePeers>,
    peer_route_index: SingletonTransactional<PeerRouteIndex>,
    // ...
}

for x in txn.peers.iter() {
    for y in txn.best_paths.iter() {
         if blah {
             txn.active_peers.insert(my_new_value);
         }
    }
}