Clean up rate computation utility.
Remove pressure from the allocator.
Be more consistent with naming.

The Persistent concept does not yet exist as part of an interface, but all
persistent classes offer a public member function save(), load(), and
potentially scan(). Eventually we should codify this concept somehow.
More statistics during ingestion.
Bring back vectors and set.

We need these types at least at the logical level to distinguish them from
records.
Fix result<T> inefficiencies.
Streamline ingestion process.

This commit fixes incorrect processing of delayed segment ACKs, Bro 2.x
container type parsing, and improves the interface for synchronous event
processing.
Fix indexing of values.

We now completely forego container types during the indexing phase and delay
the index construction of empty values until we hit the first non-empty one.