Add a block pruner that only prunes occassionally

[cjepson]

The blockchain pruning functions would run at every newly inserted
block node, causing performance degradation. Now the blockchain will
only prune nodes to return the memory to the garbage collector at
prespecified (5 minute) intervals, resulting in much less overhead
when syncing the blockchain.

[jrick]

Haven't done a full review but the code appears correct. Only issue I have is this feature is mandatory (probably required to prevent leaks?) and not configurable at all (one might want to run it more often, or based on the number of nodes in memory rather than on a timer).

[alexlyp]

synced testnet in approx 22min, mainnet in 16 min. running smooth otherwise, no noticeable increase in mem usage either

[cjepson]

@jrick I agree it's not the most ideal method, but there is a constant to change if you'd like to for the number of minutes to wait before pruning. It can be improved in the future, but for now I'd like it merged because the syncing improvement is so dramatic.

[jrick]

Work can still be done to make this more flexible for callers but for the common case (usage in dcrd) this improves IBD quite a bit.

[cjepson]

Thanks, going in