[Enhancement] Improve locking for router and routing rule processing

[er1cthe0ne]

In aca_ovs_l3_programmer.cpp, we are using a global level mutex to lock the access to our routers_tables:

  // -----critical section starts-----
  _routers_table_mutex.lock();
  _routers_table.emplace(router_id, new_subnet_routing_tables);
  _routers_table_mutex.unlock();
  // -----critical section ends-----

We can improve the locking the use read/write locks, and also add locks into lower level table entries like subnet_routing_tables and routing_rules to avoid multiple threading issues.

References:

https://en.cppreference.com/w/cpp/container#Thread_safety
https://github.com/kshk123/hashMap