Stop loading most of the BE while compiling CLJS

Nobody has claimed this yet.

While compiling CLJS, we accidentally end up loading most of the Metabase BE:

- Compiling CLJS means running macros.
- The macros are written in JVM Clojure, and run inside the CLJS compiler.
- So CLJS compilation begins by loading the same code (more or less) in JVM Clojure.
- That accidentally includes a large chunk of Metabase's BE code, including parts that try to start DB connections among much else.

Rearranging some namespaces and requires, and/or making the expensive things a bit lazier, should be able to reduce that startup time greatly. I don't have a good dependency graph, but I got a good picture of which namespaces (transitively) take a long time to load using [this Clojure patch](https://gist.github.com/pesterhazy/7f8af5701b51ccd86f99fc222436e6ea) which logs each namespace's load time. `metabase.util` (and in particular its `metabase.util.jvm` that it re-exports in JVM Clojure) are particularly suspect.

Start by applying the linked Clojure namespace-load-time logging patch and inspect metabase.util, especially metabase.util.jvm, during CLJS compilation. Trace the namespaces that pull in backend code and database startup; done means CLJS compilation no longer loads most of the BE and startup time is greatly reduced.

Written by the indexing model from the issue text.