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
This commit adds support for relaying blocks between peers. It keeps
track of inventory that has either already been advertised to remote peers
or advertised by remote peers using a size-limited most recently used
cache. This helps avoid relaying inventory the peer already knows as
much as possible while not allowing rogue peers to eat up arbitrary
amounts of memory with bogus inventory.
