Use folsom for repl stats

[russelldb]

depends on

basho/folsom#4
basho/folsom#3
basho/folsom#2

and

boundary/folsom#22

[buddhisthead]

when I add a listener to the replication master, the node crashes due to a bad function call:

2012-06-05 12:36:25.400 [error] <0.2302.0> gen_server riak_repl_leader_gs terminated with reason: bad argument in call to ets:update_counter(folsom_counters, {riak_repl,elections_leader_changed}, 1) in folsom_metrics_counter:inc/2 line 45

If I am not mistaken, there is a typo in ./deps/riak_repl/src/riak_repl_stats.erl.
"increment_counter(elections_leader_changed)." does not agree with "{elections_leader_change, counter}," by the very tiny single character "d"...change vs changed.

I changed the code to use elections_leader_changed consistently and that resolved the crash.

[russelldb]

Added a commit and some much needed eunit tests.

[buddhisthead]

+1 to merge. Good fix and unit tests. Thanks!