Fix map access and pointer deletion bugs; improve read concurrency with shared locks.

[mulugetam]

This update fixes two bugs and adds one performance improvement:

1. Avoids the use of operator[] when accessing std::map, which can unintentionally insert default values.
2. Ensures that the delete operator is called only for pointer types.
3. Improves performance by using std::shared_lock<std::shared_mutex> instead of std::unique_lock<std::shared_mutex>, allowing concurrent read access.
4. Adds tests for ShardedMap.

[asonje]

could you add tests where possible

[mulugetam]

Looks like ShardedMap doesn’t have any test coverage yet. I’ll check it out and add some.