This allows removing completely the '_node_get_option_str' on context
and ensures every method relying on a few set of keys set their errors
consistently

Pave the way toward supporting the 'spawn' method of creating jobs, by
adding support for pickling ChildJobs. Introduce a new 'jobpickler'
module that provides an entrypoint for this functionality.

This also makes replays of jobs possible, which has made the debugging
of plugins much easier for me.

This reverts one of the changes in
https://gitlab.com/BuildStream/buildstream/commit/fa70a4299bc97b1f723a8602f6fad1bcc013f17a#11743b796594142e47df22054b99d263d52e28aa_1151_1152
and will be followed in a new issue.

add name to sources in cache key dict

All of the errors which subclass from BstError have their first
positional argument as message, LoadError should follow this
ordering for consistency.

This allows us to remove the platform reset helpers in
tests/conftest.py.

`mapping.get_sequence(...).as_str_list()` is a very common
pattern seen both in plugins and the core. Adding a helper to reduce
the number of operations will make usage smoother

Currently Element._cached_success() will call two functions
to determine whether an Element is already cached. Now, we
set __cached_successfully to True once we determine that the
Element is cached for the first time. This will then short-circuit
upon (very frequent) future calls.