Change selfavatar type to BlobObject

This changes the type of ConfigItem::Selfavatar to a BlobObject.  This
is what also happened on master because there was a bug with how
selfavatar was not being correctly handled as a blob.

As a side-effect this also adds a lifetime to the ConfigItem object.
This resulted in some strum derives no longer working which in itself
resulted in some simplifications between ConfigKey and ConfigItem:

ConfigKey::to_key_string & ConfigKey::from_key_string are used to
create the SQL keynames.  The ConfigItem is converted to its
ConfigKey discriminant in the SQL write path which avoids the
duplicate source for SQL keyname.

FFI-level tests are added for testing the copy behaviour since that is
now effectively a problem of the FFI, in Rust this is impossible to
have thanks to the types.

python/tox.ini

@@ -55,7 +55,6 @@ deps =
 commands =
     sphinx-build -w toxdoc-warnings.log -b html . _build/html
 
-
 [testenv:lintdoc]
 skipsdist = True
 usedevelop = True
@@ -66,14 +65,12 @@ commands =
     {[testenv:lint]commands}
     {[testenv:doc]commands}
 
-
-
 [pytest]
-addopts = -v -rs 
-python_files = tests/test_*.py 
-norecursedirs = .tox 
+addopts = -v -ra
+python_files = tests/test_*.py
+norecursedirs = .tox
 xfail_strict=true
-timeout = 60 
+timeout = 60
 timeout_method = thread
 
 [flake8]