Add a capability to guard riak_core_net_ticktime

The goal of this change is to prevent problems in a mixed version
cluster where the riak_core_net_ticktime module is not loaded. The
capability is net_ticktime and has two modes: supported and
unsupported. The supported mode is the preferred mode. The
net_ticktime daemon will not run until the capability has a mode of
supported.

This change also adds `riak_core_net_ticktime:enable/0` that can be
used to register the net_ticktime capability after the
riak_core_net_ticktime module is loaded on a node.
User iterator prefetching in hashtree module

Use prefetch and prefetch_stop actions for iterator_move calls in
the hashtree module. This replaces the use of the next action with the
goal of making segment traversal more efficient. This change adds a
prefetch field to the itr_state record and some additional case
clauses in the iterate/2 function. This ensures that an iterator can
be safely used with other actions after being used for prefetch
iteration.
Only check capability on initial call to start_set_net_ticktime_daemon. Also change values of capability to boolean.
Add function to wrap capability check
plumb through coverage queries during overload
Cuttlefish schema for background manager global kill/enable switch
Rename background manager setting and add schema tests.
Fix up the doc string for background_manager.
add {refresh_iterator, true} for backend folds during handoff send
cleanup and overload handoff_started

* require options that are used in the subsequently sent ?FOLD_REQ to be
returned by the optional callback
* no longer pass SrcNode, its information the vnode can obtain elsewhere and
isn't very useful
* normalize error return values
{level, advanced} -> hidden
Merge pull request #497 from basho/cet-bg-mgr-schema

Cuttlefish schema for background manager global kill/enable switch
Merge pull request #513 from basho/ajs-overload-plumbing

plumb through coverage queries during overload
Treat security users and groups as distinct concepts with separate cluster metadata storage.
Merge pull request #534 from basho/jrd-security-grant-namespace

Treat users and groups as distinct concepts
Merge pull request #520 from basho/ajs-iterator-refresh

add {refresh_iterator, true} for backend folds during handoff send
Merge branch 'feature/hashtree-prefetch-iterator' into develop
Fix unmatched returns in hashtree
Export chashbin type, rename preflist2 to preflist_ann, and fix incorrect annotations.
Fix unknown types caused by typos