Adapt proxy heuristic to context flattening

Context

The structure of the storage in alpha was changed by !2771 (merged). This flattening had to be reflected in the --mode proxy and tezos-proxy-server heuristics for getting a parent piece of the context (aka the split_key heuristic). For some paths within the low-level storage, this heuristic makes the proxy mode requests /A/B/C instead of /A/B/C/x, in order to batch requests (supposing that, in this example, there will be a sequence of requests of the form /A/B/C/x, /A/B/C/y, /A/B/C/z; so better request /A/B/C at once).

This MR adapts the heuristic and adds a tezt that the heuristic works for big_maps requests of tezos-proxy-server. I did not add correspondig tests for --mode proxy of tezos-client because it required more work (as the client doesn't have the support for events logging in tezt).

Manually testing the MR

• Check that the changes to src/proto_alpha/lib_client/proxy.ml match the changes to paths described in !2771 (merged).
• alias tezt='dune exec tezt/tests/main.exe --'
• tezt --file proxy_server_test.ml --test 'Alpha: big_map_perf (proxy_server)' --verbose
• tezt --file proxy.ml --test 'Alpha: (Proxy) split_key heuristic' --verbose

Checklist

• NA Document the interface of any function added or modified (see the coding guidelines)
• NA Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• NA For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
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