Connection pools documentation and example

[MeanSquaredError]

This PR adds:

• A documentation file for connection pools docs/Connection-Pools.md
• The documentation on threads is slightly expanded.
• A basic example which shows how to workaround the multithreading issues using connection pools. The sample program creates a thread_local wrapper for a connection handle. The thread_local wrapper exposes all relevant connection methods, in this example it exposes just operator(). The wrapper lazily fetches a connection from the pool the first time when operator() is called. Then the example runs 10 thread, each thread running 5 SQL queries in a loop.

[rbock]

Thanks for the documentation and example!

[rbock]

Thanks for documenting and providing examples! Nice work!