Fix EdgePropertyList construction via _from_object_info

[greschd]

Fixes #561

The EdgePropertyList retains the same list instance during its lifetime
for containing the edge property wrappers. This is done so that references
user code might hold to its items to correctly update its contents.

In contrast to other types, it means that the state is not re-fetched from
the _pb_object if it were to change outside of the EdgePropertyList's
control.
This led to a bug when constructing objects via _from_object_info, which
first default-constructs the object (in the process creating the EdgePropertyList)
and then modifies the _pb_object.

To fix this, the EdgePropertyList now stores whether its parent object was
stored when this list was last accessed. If it changes from unstored to stored,
the following logic is applied:

• If the current list is already populated (the "regular" case), only a sanity check
  for matching length is performed
• If the current list is empty, it is re-fetched from the parent object. This is the
  case which occurs during construction with _from_object_info

Since the edge property list can be in an inconsistent state (empty when it shouldn't
be) while the parent is unstored, we disallow accessing it in this state. It is still
allowed however to fully replace the contents.

This PR also fixes a bug in tree_object_from_resource_path, where the channel
argument was still used instead of server_wrapper.

[greschd]

@roosre this is a fairly significant bugfix, I'd suggest we do a 0.1b2 release for it (and only drop Python 3.9 support after that).

[codecov]

Codecov Report

Attention: Patch coverage is 98.03922% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.91%. Comparing base (eb5056c) to head (661edaf).
Report is 2 commits behind head on main.

Files	Patch %	Lines
...ore/_tree_objects/_grpc_helpers/property_helper.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #562      +/-   ##
==========================================
+ Coverage   91.93%   92.91%   +0.97%     
==========================================
  Files          82       82              
  Lines        4242     4291      +49     
==========================================
+ Hits         3900     3987      +87     
+ Misses        342      304      -38     

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