Use cluster metadata to get/set transfer limits

riak_core_handoff_manager now uses cluster metadata to get and set
transfer limits per node. The configuration in the file overwrites the
current cluster metadata value for that node on startup of the handoff
manager. All sets still use RPC calls to the given node, but then add
them to cluster metadata instead of changing the configured environment
variable. The environment variable now always matches what was in the
config file at node startup.
Add rpc versions of transfer limit

Add backend calls for `riak-admin transfers limit` that use rpc like the
old version of `riak-admin transfer-limit`. The versions intended to be
used without a flag use cluster metadata for efficiency.
New CLI -  riak-admin handoff limit <action> <value>

 * Flag `--node` will operate on the given node
 * Without passing `--node` all nodes operated upon
 * `riak-admin handoff` lists subcommmands (limit only for now)
 * `riak-admin handoff limit` shows usage for limit subcommand
 * Add rpc versions of `riak-admin handoff limit --show` when using
   --force-rpc option.
Fix comment formatting (double-% FTW)
Merge pull request #640 from basho/feature/dtr/handoff-new-cli-1237

New CLI -  riak-admin handoff limit

Reviewed-by: macintux
Merge pull request #637 from basho/feature/metadata-backed-transfer-limits

Use cluster metadata to get/set transfer limits

Reviewed-by: macintux