Multithreading support

[paulin-mipt]

A known issue with Google API is Http() objects being not thread safe (https://github.com/googleapis/google-api-python-client/blob/main/docs/thread_safety.md).

For my usecase (a multithreaded chat bot interacting with Spreadsheets heavily) it appears as various SSL errors during sheet access and occasional python core dumps.

So far I've done a quick dirty fix which might eventually grow up to PR here, but just wanted to leave it as a note in case you folks know a better way to handle multithreading. Your package is way too convenient to just drop it because of concurrency issues :)