PBS-6 feature: Rework storage layer abstractions to support simultaneous reads and writes (part 3)

[percona-ysorokin]

https://perconadev.atlassian.net/browse/PBS-6

Implemented the most straightforward locking strategy for 'binsrv::storage_core'.

• Added an instance of the 'std::shared_mutex' to the 'binsrv::storage_core' class.
• Public const methods are now protected with 'std::shared_lock'.
• Public mutable methods are now protected with 'std::unique_lock'.
• For public methods that are called from other public methods introduced its 'xxx_unsafe()' version to avoid double locking.
• Removed 'noexcept' qualifier from methods that create 'std::unique_lock' / 'std::shared_lock'.
• Public const methods that return data that is initialized only once during object construction left without locks.
• Some methods that used to return references to internal objects now return by value to achieve thread-safety guarantees.