Investigate how to support the use-cases from indexpiration

Currently, expirationd does not support iteration over a (secondary) index (since the expriration check callback must be applied to each tuple from a space). This creates a situation where a full scan is applied instead of selecting tuples from some start index value.

TODO:

• Investigate how indices are used in the current expirationd implementation (check how the primary index is used, is it possible to iterate over a complex primary index, simple or complex secondary index)
• Propose a RFC for changing the API to support iterating over a specified index or a field (the minimal satisfying index must be chosen in this case if it exists)
• Check how the module is initialized: is it possible to reload the configuration and any created fibers or the internal state at runtime, thus supporting the "hot reload" feature
• Propose a RFC for changes to support the "hot reload" feature