Export stats

[theoreticalbts]

This code implements two plugins:

• block_data_export_plugin allows export of per-block data; it effectively implements Routing per-block data #2244.
• stats_export_plugin uses the interface provided by block_data_export_plugin to dump GPO and per-user bytes per block. The intent is that this data will be consumed by a Python script for further processing.

The data path implemented by block_data_export_plugin will be extremely useful going forward, especially when combined with a UNIX FIFO connected to some external script.

I also implemented a simple thread pool to allow all available processor cores to be utilized for the JSON conversion of exported data objects. Unfortunately Boost's thread pools aren't suitable, because (1) they're marked experimental and may be subject to change in future Boost versions, and (2) there's no way to set the thread stack size, which we need because FC JSON processing uses stack very heavily.

When processing the main blockchain, approximately 40 GB of stats are produced. To minimize disk usage, you can do streaming compression of the output JSON by simply using a UNIX FIFO connected to pbzip2 or similar.

[mvandeberg]

Because you are using the block_data_export_plugin as a dependency, you should specify it in APPBASE_PLUGIN_REQUIRES

[mvandeberg]

I only reviewed commits e548eae and 9354983 because the others are included in #2325