feature: Support setting size of LRU-cache

Fixes #10.

A few comments:

* This seem to work - but I haven't found a good way to test that it actually works
* I've tried running the benchmarks with different cache-settings without that effecting the performance at all - I guess that this is because the data read is in the memtable and is therefore already in memory. Any suggestions on how a benchmark where we can see some performance gain from using the LRUCache would be highly appreciated
* There are no unit tests for this. I don't really know how to test it.
* For simplicity the `fillCache` defaults to *true* when running get() and *false* when running readStream()