Provide support for Gevent

[Gevent](http://www.gevent.org/) is basically used to make parallel network calls. We are using gevent in one of the transformation methods to call internal services. Before using the gevent we also patch it as mentioned [here](https://github.com/grpc/grpc/issues/4629#issuecomment-376962677).
The transformation method is making multiple network call in parallel. Here is the code snippet:
```

/__init__.py
import gevent.monkey
gevent.monkey.patch_all()

/transform.py
from gevent import
Greenlet
from gevent import joinall
def filter_out_invalid_users(events):
key, user_id_data_pairs
= events
user_ids = [user_id for user_id, data in user_id_data_pairs]

jobs = []
id_chunks
= utils.chunk_list_evenly(user_ids, BATCH_SIZE)
for id_chunk in id_chunks:
jobs.append(Greenlet.spawn(_call_users_service,
# _call_user_service_ method is making the network call.
list(id_chunk)))

"""
Here we
increase the timeout based on the number of greenlets we are running, to account for yielding
among
greenlets
"""
join_timeout = GREENLET_TIMEOUT + len(jobs) * GREENLET_TIMEOUT * 0.1
joinall(jobs,
timeout=join_timeout)

successful_jobs = [job for job in jobs if job.successful()]
valid_user_ids
= []
for job in successful_jobs:
network_response = job.get()
valid_user_ids.append(network_response.user_id)

yield valid_user_ids

def _call_users_service(user_ids):
# make network call and return
response
..
..
return network_response

```

This allows pipelines to start and partially run. However the tasks produce a stream of gevent exceptions and do not make any progress.:
```

 
Exception greenlet.error: error('cannot switch to a different thread',) in 'grpc._cython.cygrpc.run_loop'
ignored
Traceback (most recent call last):
  File "src/gevent/event.py", line 240, in gevent._event.Event.wait
 
File "src/gevent/event.py", line 140, in gevent._event._AbstractLinkable._wait
  File "src/gevent/event.py",
line 117, in gevent._event._AbstractLinkable._wait_core
  File "src/gevent/event.py", line 119, in gevent._event._AbstractLinkable._wait_core
 
File "src/gevent/_greenlet_primitives.py", line 59, in gevent.__greenlet_primitives.SwitchOutGreenletWithLoop.switch
 
File "src/gevent/_greenlet_primitives.py", line 59, in gevent.__greenlet_primitives.SwitchOutGreenletWithLoop.switch
 
File "src/gevent/_greenlet_primitives.py", line 63, in gevent.__greenlet_primitives.SwitchOutGreenletWithLoop.switch
 
File "src/gevent/__greenlet_primitives.pxd", line 35, in gevent.__greenlet_primitives._greenlet_switch
greenlet.error:
cannot switch to a different thread
 
```

The alternative approach is to use multiprocess module as shown [here](https://github.com/apache/beam/blob/59c85b44d156bb7b4462d80fcb5591f860235708/sdks/python/apache_beam/internal/util.py#L117) & [here](https://github.com/apache/beam/blob/7bd73a51b670755bbb19e1291003722d5d16bdc5/sdks/python/apache_beam/io/filebasedsink.py#L313).

Gevent is lightweight and good for parallelizing IO/Network bound jobs and it can efficiently use resources and it can scale well in case heavy load.
 

Imported from Jira [BEAM-5497](https://issues.apache.org/jira/browse/BEAM-5497). Original Jira may contain additional context.
Reported by: rakeshkumar.