Add cache for immutable segments for commonly loaded fields

[black-adder]

There's a common use case where the same field can be loaded from disk multiple times in succession. Right now, we will fetch the data from disk for each query.

Before we go ahead and build a cache, since fields are memory mapped on the first read, write a benchmark that checks if subsequent reads take advantage of the memory map and what the performance is. If the performance is "good", a cache may not be required.