Add Scheduler.shield() and Scheduler.wait_and_close()

[Dreamsorcerer]

Fixes #48. Fixes #99.

Needs:

• Documentation changes
• Tests

Basic example of this:

import asyncio
from contextlib import suppress

import aiojobs

async def important():
    print("START")
    await asyncio.sleep(5)
    print("DONE")

async def run_something(scheduler):
    # If we use asyncio.shield() here, then the task doesn't complete and DONE is never printed.
    await scheduler.shield(important())

async def main():
    scheduler = aiojobs.Scheduler()
    t = asyncio.create_task(run_something(scheduler))
    await asyncio.sleep(0.1)
    t.cancel()
    with suppress(asyncio.CancelledError):
        await t
    await scheduler.wait_and_close()

asyncio.run(main())

[codecov]

Codecov Report

Attention: Patch coverage is 94.80519% with 8 lines in your changes missing coverage. Please review.

Project coverage is 97.15%. Comparing base (8b085d4) to head (61c80a4).

Files	Patch %	Lines
aiojobs/_scheduler.py	80.95%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #495      +/-   ##
==========================================
- Coverage   97.56%   97.15%   -0.41%     
==========================================
  Files           8        8              
  Lines         903     1054     +151     
  Branches       78       91      +13     
==========================================
+ Hits          881     1024     +143     
- Misses         14       18       +4     
- Partials        8       12       +4     

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