1. Add ExternalAllocator that provides files ranges to write to.
2. Add IoMgr that wraps files and allows sending asynchronous requests.
3. Add POC that writes string values when a new entry is added.
The original values kept are still kept in memory.
1. Added support of loading of compressed strings.
2. Verified we load expiry info.
3. Extended supported expiry period to 4 years (previously I set 1 year).
1. Sets do not use anymore redis encoding constants in our codebase.
2. rdb_save serializes now expiry information as well.
3. db_slice provides a simplified api to translate from absolute time to ExpirePeriod.
4. common_types.h is renamed to common.h. cached memory usage variables are moved there.
