RFC: use LB3 models in an LB4 app [SPIKE - DO NOT MERGE]

SPIKE-NOTES.md

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+## Open questions
+
+- How to test code migrated from strong-remoting and loopback (v3)? Do we want
+  to copy existing tests over? Migrate them to async/await style? Don't bother
+  with testing at all, use few acceptance-level tests only?
+
+- How to split 2k+ lines of new (migrated) code into smaller chunks that will be
+  easier to review?
+
+- Should we register LB3 Models for dependency injection into LB4 code? Register
+  them as repositories, models, services, or something else?
+
+- Should we implement booting datasources from `datasources.json`? I see two
+  obstacles:
+
+  1. Datasources must be booted before models are attached.
+  2. LB3 supports variable replacement in JSON config files using ENV vars but
+     also `app.get(name)`. This may add unwanted complexity to our compat layer.
+
+## By the way
+
+- TypeScript does not support static index signature, which make it difficult to
+  consume custom model methods. See
+  https://github.com/Microsoft/TypeScript/issues/6480.
+
+- I'd like us to extract the Booter contract into a standalone package so that
+  v3compat and other similar extensions don't have to inherit entire boot and
+  don't have to lock down a specific semver-major version of boot in extension
+  dependencies. Instead, they should depend on the Booter contract only, this
+  contract should not be changed so often (hopefully).