InMemoryChannel Dispatcher refactor

[nachocano]

Fixes #1892
Helps with #1693

Proposed Changes

• Using cloudEvents SDK instead of "plain" HTTPServer in fanout, multichannelfanout, and swappable.

• Renaming MessageDispatcher and MessageReceiver to EventDispatcher and EventReceiver

• Updating UTs.

• Note that event_dispatcher and event_receiver are renamed files from message_dispatcher and message_receiver. Most of the code is from them. Not sure why github doesn't mark them as renames. Same happened with some UTs

Release Note

NONE

[nachocano]

/retest

[Harwayne]

/lgtm
/approve
/hold

Please make the TODOs into issues, the cancel the hold.

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Harwayne, nachocano

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[nachocano]

/hold cancel

[nachocano]

@Harwayne need the magic lgtm again :)

[knative-metrics-robot]

The following is the coverage report on pkg/.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/channel/event_dispatcher.go	Do not exist	82.6%
pkg/channel/event_receiver.go	Do not exist	64.2%
pkg/channel/fanout/fanout_handler.go	96.2%	84.6%	-11.5
pkg/channel/history.go	Do not exist	100.0%
pkg/inmemorychannel/dispatcher.go	18.8%	22.2%	3.5
pkg/utils/context.go	Do not exist	0.0%

[knative-prow-robot]

@nachocano: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-knative-eventing-go-coverage	51ca89b	link	/test pull-knative-eventing-go-coverage

Full PR test history. Your PR dashboard.

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[Harwayne]

/lgtm