get returns remaining ttl

[buster1253]

lru.get() returns the remaining ttl on success. Comparable to ngx.shared.DICT.ttl altough integrated directly into the get function.
Could change it to a standalone function if that's preferable.

[thibaultcha]

:get() now already has other return values. We should follow the lua_shared_dict API and implement this as a standalone :ttl() method (i.e. ngx.shared.DICT.ttl).