* Remove 32bit os support from `custom_network` unit test

also:
* Fix compilation annoyance #1648
* Fix unit test on Kiln (changed `merge` logic?)

* Hide unused sources do not compile

why:
* Get them out of the way before major update
* Import and function prototype mismatch -- maybe some changes got out
  of scope.

* Re-implemented `db_chain` as `core_db`

why:
  Hiding `TrieDatabaseRef` and `HexaryTrie` by default allows to replace
  the current db wrapper by some other one, e.g. Aristo

* Support compiler exception warnings for CoreDbRef base methods.

* Allow `pairs()` iterator on all memory based key-value tables

why:
  Previously only available for capture recorder.

* Backport `chain_db.nim` changes into its re-implementation `core_apps.nim`

* Fix exception annotation

why:
* Resolves some compiler coughing when it bails out on persitent
  db constructor inside `test()` caluses (works perfectly outside.)
* API looks cleaner and better to maintain for the price of slightly
  more work at the backend

available tracers:
- Legacy tracer
- new Json tracer