Deprecated old style path+data JSON query request format

[mina86]

#7601 removed an undocumented format for doing RPC query requests but turns out some clients were still using it so it got reverted in #7797. We might want to come up with some plan of deprecating that format and getting rid of the code.

[matklad]

Two thoughts here:

• we might actually leave it be? This is surface-only functionality, so it's not like we gain a lot from actually removing support here.
• we can add probabalistic failures: eg, fail 0.1% of requests with a clear error message, ramp that up over sereval releses to 50%, then remove.

Retrospectively and as a lesson for the future, #7601 should probably have kept the parsing code in to return a clear error message.