[forwarder] allow multiple endpoints/api_keys

[degemer]

[config] remove disable_dd CLI option

It simplifies the config logic.

[forwarder] allow multiple endpoints

In order to do so, the config was modified:
other_dd_urls and other_api_keys were added.
They both accept a a comma-separated "list", which is
transformed in a endpoints dict. (merged with dd_url and api_key)
This one has the following format:
{
'https://app.datadoghq.com': ['api_key_abc', 'api_key_def'],
'https://app.example.com': ['api_key_xyz']
}

The forwarder is the only one using this new configuration.
It loops through all endpoints and sends the same payload.

Config tests and update transaction tests.
Set default max_parallelism to 1 in transaction.py. (it's the real
default)

[forwarder] handle errors for each endpoint

Previously everything was dismissed except for the last result.
This commit duplicates payload for each endpoint, so that the transaction
manager handles them properly.

Another solution would have been to make the transaction aware of the errors
for each of the endpoints, but the Transaction Manager and the Transaction were
not created to do this, and thus is/would be a huge pain to update them.
(because of the async nature of the flush)

[forwarder] enable multiple parallel requests

Throttling is still there, so this should only improve the situation where
there is a long running request (especially for mutliple endpoints, if one of
them is timeouting, this should improve the situation of the other endpoint)

[forwarder] add a configurable timeout

Default timeout is 20s, same as before (it's the default timeout of
tornado).
It is now possible to configure it.

[forwarder] limit errors per endpoint per flush

When an endpoint fails 4 times, it is considered down (or at least
unreachable), and all transactions targetting this endpoint are dropped
from the current flush. (they are rescheduled for a future flush)

This also changes the way transactions are scheduled in each flush: new
transactions without any error will be tried first.

It also cleans up a little logs.

[gmmeyer]

How do I set a second endpoint and associate keys with it? (I can see how to do it in the tests, you just don't have it here.)

[degemer]

Indeed, it's just that our customers shouldn't be using it. But you're right, it should be specified anyway. I added it. ✅

[gmmeyer]

I think this looks good, though we definitely want to test it extensively to be sure there's no memory issues. 👍

[degemer]

Thanks for the review @gmmeyer! I'll be sure to double check this before releasing.