Proposal: change get-* endpoints that use HTTP post

Background
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Right now we HTTP POST ASCII key-value pairs on these endpoints:
	* get-leaves
	* get-inclusion-proof
	* get-consistency-proof

The original reason was to not have an additional parser, say, input-parameters
with percent-encoding as part of the request URL.

A major problem with this approach is that it will not be possible to benefit
from HTTP caching.  Debugging, with "URLs that reference data" also becomes more
messy.  You would have to say "I did printf <stuff> | curl ...".

Proposal
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Change these endpoints so that they use HTTP GET.  Encode input params in URL:

	<url>/get-leaves/10/20 # get leaves 10,11,...20
	<url>/get-consistency-proof/10/20 # proof from tree size 10 to 20
	<url>/get-inclusion-proof/10/<leaf hash in hex> # proof for tree size 10

This notably avoids percent-encoding which is more messy.

Notes
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We considered if it would be a good idea to re-use our ASCII parser for the
portion of the URL that encodes input data.  The basic idea would be that
different "end of key" and "end of value" patterns could be used that are better
suited for a URL.

For example, instead of (=,\n) one could use ([,]) as ("end of key", "end of value").
	* get-leaves/start_size[10]end_size[12]
	* get-consistency-proof/old_size[12]new_size[14]
	* get-inclusion-proof/tree_size[10]leaf_hash[ab...ef]

The reasons why we aborted this direction:
	* We can not think of any concrete security risk with the shorter '/' proposal.
	* There are very few parameters at play here, hard to confuse and quick
	feedback loop if you do. For example, "Error=start size must be smaller or
	equal to end size".
	* We can be sure that the '/' proposal will not introduce any wonky
	interoperability issues; picking a ("end of key", "end of value") would
	require much more care.