rpc: remove dependency of URL (GET) requests on tmjson (#7590)

The parameters for RPC GET requests are parsed from query arguments in the
request URL. Rework this code to remove the need for tmjson. The structure of a
call still requires reflection, and still works the same way as before, but the
code structure has been simplified and cleaned up a bit.

Points of note:

- Consolidate handling of pointer types, so we only need to dereference once.
- Reduce the number of allocations of reflective types.
- Report errors for unsupported types rather than returning untyped nil.

Update the tests as well. There was one test case that checked for an error on
a behaviour the OpenAPI docs explicitly demonstrates as supported, so I fixed
that test case, and also added some new ones for cases that weren't checked.

Related:

* Update e2e base Go image to 1.17 (to match config).

test/e2e/docker/Dockerfile

@@ -1,7 +1,7 @@
 # We need to build in a Linux environment to support C libraries, e.g. RocksDB.
 # We use Debian instead of Alpine, so that we can use binary database packages
 # instead of spending time compiling them.
-FROM golang:1.16
+FROM golang:1.17
 
 RUN apt-get -qq update -y && apt-get -qq upgrade -y >/dev/null
 RUN apt-get -qq install -y libleveldb-dev librocksdb-dev >/dev/null