Add a @client_callable decorator

which auto-exports functions on (SQL) classes
Encoder: keep a copy of the data around
minor import cleanup
grab the UTF8-clean prettyprinter from Pybble

I need it for debugging, at least
Implement a Cloning Local Manager

DaBroker's forking primitives copy the content of the cloning
managers to the new task's local storage.

This helps with maintaining an established context,
e.g. having multiple clients and/or servers in one Python process.
classmethod mod, test
Format data in encoding error messages sensibly
fix codec
redundant naming
misplaced code
fix string repr
Renamed Thread.run to _run

conflicts with Main.run
overriders should use .code
Fix coding of errors

don't call the derived group's codec on nested structures
as some may not like to enclode their own output
rename the exchange' config parameter to 'rpc_exchange'
Properly tag methods as "exported"

and distinguish between class methods on the exported object
(e.g. the SQL table) vs. on the DaBroker metaclass (like _dab_search)

and test all of that
more documentation
improve error message
remove spurious debugger call
Drop 'exported_method', unnecessary
pass IndexError