app-layer: more generic state trait

app-layer: more generic state trait

Instead of a method that is required to return a slice of transactions,
use 2 methods, one to return the number of transactions in the
collection, and another to get a transaction by its index in the
collection.

This allows for the transaction collection to not be a contiguous array
and instead can be a VecDeque, or possibly another collection type that
supports retrieval by index.

Ticket #5278