Send subscription messages in order

[dceleda]

Fixes #3457
Partially Fixes #3444

Changes:

Subscription messages will be queued up and sent in order.

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Other (please describe):

Testing

Requires testing

• Yes
• No

In case you checked yes, did you write tests??

• Yes
• No

[LukaszRozmej]

Not sure about performance considerations of scheduling tasks as well as creating a channel per each subscription

[dceleda]

Not sure about performance considerations of scheduling tasks as well as creating a channel per each subscription

We've been creating tasks there anyway so I just queued them up.
Channels should perform better than blockingcollection or other structures.

We could potentially optimize it. For example use a dedicated task scheduler and runsynchronously on it.