Improve performance with smarter caching

[danvk]

Fixes #44

This improves performance enough for the genes + reference genome track to be pleasant to use (~30fps, was ~1fps before).

Highlights:

• Fetches 2bit data in blocks of 1000 base pairs. Previously w'd request 1200-1700, then 1199-1699, then 1198-1698, etc. This generated a lot of unnecessary network requests.
• Fetches & caches entire blocks of Ensembl data at once. This data was previously cached by RemoteFile, but it turns out that decompression is expensive.

The second tweak was the big performance win.

[coveralls]

Coverage increased (+3.97%) to 67.26% when pulling 9f7d45e on perf into 6a58a5c on master.

[coveralls]

Coverage increased (+4.19%) to 67.48% when pulling 616c3c1 on perf into 6a58a5c on master.