gdss-brick >> gh17 Redesign disk storage:
- Introduce brick_metrics module, a folsom-based metric system
  to generate histograms of latency and operation counts.
  Currently, the following metrics per node (not a brick or table)
  are available:
  * squidflash priming latency
  * logging op latency (do-ops waiting for a group commit in WAL)
  * WAL sync latency
  * WAL sync requests (for now, number of brick servers, not do ops)

Details:
- brick_metrics:
  * Add this thin wrapper module of boundary/folsom.
- brick_ets:
  * Add the following metrics:
    ** squidflash_priming_latency
    ** logging_op_latency
  * Remove old logging_op statistics named "sync summary"
    ** logging op count
    ** logging op average latency
  * Remove some debug prints.
  * Replace some erlang:now/0 occurrences with os:timestamp/0.
    (Leave some occurrences where the uniqueness and continuously
    increasing timestamps are important.)
- gmt_hlog:
  * Add the following metrics:
    * WAL sync latency
    * WAL sync requests
  * Replace some erlang:now/0 occurrences with os:timestamp/0.
  * Remove some debug prints.
- brick_server and gmt_hlog_common:
  * Replace some erlang:now/0 occurrences with os:timestamp/0.