Enable memdb support.
Vastly optimize the MRU inventory handling.

Profiling showed the MRU inventory handling was taking 5% of the total
block handling time.  Upon inspection this is because the original
implementation was extremely inefficient using iteration to find the
oldest node for eviction.

This commit reworks it to use a map and list in order to achieve close to
O(1) performance for lookups, insertion, and eviction.

The following benchmark results show the difference:

Before: BenchmarkMruInventoryList          100     1069168 ns/op
After:  BenchmarkMruInventoryList     10000000         152 ns/op

Closes #21