uses lazy LRU cache for ReedSolomonCache

[behzadnouri]

Problem

Lazy LRU is generally faster and allows immutable shared access on the read path.

Summary of Changes

• The commit uses lazy LRU cache for ReedSolomonCache.
• This also allows to use shared-locks on the read path.

[alexpyattaev]

Looks like lazy_lru is also quite a bit simpler inside (as well as not requiring exclusive locking). Lgtm. It may be a good idea to migrate to it for all use-cases.