reduce size of generated GAPICs

[vchudnov-g]

Let's figure out how we can make our GAPICs have a smaller memory footprint and be more efficient. (Googlers: related issue is b/319242502).

Some ideas:

• [ ] refactoring repeated code in the GAPICs
• [ ] refactoring API-independent code into python-api-core
  • some of this may be gated on the Python library versions we support
• [ ] lazy loading (#1268 )
• [ ] making the async client a derived class of the corresponding (sync) client. Right now the async clint defers a lot of calls to the sync client it contains.
• [ ] remove useless comments. We don't need comments when we're calling a well-named function or simple expression. (eg # Return the response for return response)