Extend per-language Getting Started examples with concurrency

[petermattis]

A forum user attempted add threading to the psycopg2 example but didn't take into account the thread safety of psycopg2 connections or the isolation of psycopg2 cursors. At a minimum, we may want to add a sentence about thread-safety (or the lack thereof) to these examples. Going farther, we could add additional per-language examples that include concurrency as we want to encourage users to utilize concurrency in their apps in order to get the best performance out of CockroachDB.

[jseldess]

Thanks, @petermattis. This is a duplicate of #2464, so I've copied your comment over there.