* Enable `snap/1` accounts range service
* Allow to change the garbage collector to `boehm` as a Makefile option.
why:
There is still an unsolved memory corruption problem that might be
related to the standard `gc`. It seemingly goes away if the `gc` is
changed to `boehm`.
Specifying another `gc` on the make level simplifies debugging and
development.
* Code cosmetics
details:
* updated exception annotations
* extracted `worker_desc.nim` from `full/worker.nim`
* etc.
* Implement option to state a sync modifier file
why:
This allows to specify extra sync type specific options which might
change over time. This file is regularly checked for updates.
* Implement a threshold when to suspend full syncing
why:
For a test scenario, a full sync beep may work as a local snap server.
There is no need to download the full block chain.
details:
The file containing the pivot specs is specified by the
`--sync-ctrl-file` option. It is regularly parsed for updates.
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