Proposal: no quick tree-head endpoint

Refer to

	https://git.sigsum.org/sigsum/tree/archive/2022-01-18-tree-head-endpoint-observations

for background.

This proposal suggests that we only have two tree head endpoints.
	1. get-tree-head-to-cosign -> signed tree head.  This is the tree head that
	witnesses are currently cosigning.  This endpoint is only meant to be used
	by witnesses.
	2. get-tree-head-cosigned -> cosigned tree head.  This is the finalized tree
	head that witnesses have finished cosigning.  The list of cosignatures is
	thus fixed.

The to-cosign and cosigned tree heads are rotated every $n minutes.  A typical
value of $n is likely going to be five (5).  It might be lower if witnesses
accept to work more.

Pros:
	* Less complex API.  It has fewer endpoints and no choice between a "quick"
	or "slow" get-tree-head-cosigned endpoint.  No choices means easier tooling,
	fewer mistakes.
	* It might be reasonable to speed up the "slow" endpoint by other means, see
	above.

It is also worth pointing out another pro that we already got from removing the
get-tree-head-latest endpoint.  A submitter is forced to wait a bit, even for a
signed tree head.  This makes it less appealing to "go with a signed tree head
because its fast".  The name of the only signed tree head endpoint also
discourages usage by submitters.