* Somewhat tighten error handling
why:
Zombie state is invoked when the current peer turns out to be useless
for further communication. While there is a chance to further talk
to a peer about another topic (aka healing) after some protocol failure,
it makes no sense to do so after a network problem.
The latter state is explained bu the `peerDegraded` flag that goes
together with the `zombie` state flag. A degraded peer is dropped
immediately.
* Remove `--sync-mode=snapCtx` option, always start snap in recovery mode
why:
No need for a snap sync option without recovery mode, can be achieved
by deleting the database.
* Code cosmetics, typos, prettify logging, debugging helper, etc.
* Split off snap sync sub-mode handler into separate modules
details:
The original `worker.nim` source has become a multiplexer for several
snap sync sub-modes `full` and `snap`. The source modules of the
incarnations of a particular sync sub-mode are places into the
`worker/play` directory.
* Update ticker for snap and full sync logging
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