Block cache lock contention

[Stebalien]

There is quite a bit of lock contention in the block cache. On the gateways, out of 32 walk threads:

• 13 were waiting on the disk.
• 19 were blocked on the cache.
  • 10 trying to add blocks
  • 9 trying to get them.

We're currently using github.com/hashicorp/golang-lru.