`utils.py`: Make `magic_timestamp` a public variable

See merge request BuildStream/buildstream!1369

Some plugin elements, such as docker-element and tar-element, require
to set `created` and/or `modified` meta-data fields. It would be nice
to use `magic_timestamp` to ensure these fields are being set in a
consistent and deterministic way.

Extract 'Messenger' from 'Context' and use directly

See merge request BuildStream/buildstream!1445

Change the argument name on the 'silence' context manager, to avoid the
interpretation that non-nested messages may be silenced, or that it can
'un-silence' (it can't).

Be more specific about what 'important' means w.r.t. messages.

Require 'actually_silence' to be specified as a keyword, so that
callsites are easier to understand.

Replace the _message_depth list with an int, "_silence_scope_depth".

Simplify by not calling _{push,pop}_message_depth outside of the silence
context manager, let that do the work instead.

Use some Python features to make Messenger._message_depth a bit simpler.

Reduce the amount of context shared with child jobs, by only sending the
messenger portion of it rather than the whole thing. Also send the
logdir.

This also means that we will need to pickle less stuff when using the
'spawn' method of multi-processing, as opposed to the 'fork' method.