Backend data expiration

[pjaclark]

Is object expiration for leveldb (https://www.tiot.jp/riak-docs/riak/kv/3.2.0/configuring/global-object-expiration/) valid?

Bitcask has specific config settings mentioned (https://www.tiot.jp/riak-docs/riak/kv/3.2.0/configuring/backend/#bitcask) for expiry, but leveldb doesn't - but leveldb has a whole page for it with options not in the config reference.

Plus, we should note what happens with TicTacAAE.

[JMercerGit]

Configuration also needs to be added here https://www.tiot.jp/riak-docs/riak/kv/3.2.0/configuring/backend/#leveldb