rgw: add "explicit_custom_meta" configurable to es sync module

rgw: add "explicit_custom_meta" configurable to es sync module

Modify all the map<string, string> that is used to pass in module config
to map<string, string, ltstr> so that it can be used with the conf get
vals util. Also, switch to using shared_ptr to hold the ElasticConfig,
so that later when we hold maps and such it doesn't need to be copied
on every call.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>