configitems: add TODOs blocking the move out of experimental for revlogv2

These are the todos so far, we probably will have more, but we might as well
list them while they're fresh in our minds.

Differential Revision: https://phab.mercurial-scm.org/D10216

mercurial/configitems.py

@@ -1297,6 +1297,14 b' coreconfigitem('
     b'use-persistent-nodemap',
     default=False,
 )
+# TODO needs to grow a docket file to at least store the last offset of the data
+# file when rewriting sidedata.
+# Will also need a way of dealing with garbage data if we allow rewriting
+# *existing* sidedata.
+# Exchange-wise, we will also need to do something more efficient than keeping
+# references to the affected revlogs, especially memory-wise when rewriting
+# sidedata.
+# Also... compress the sidedata? (this should be coming very soon)
 coreconfigitem(
     b'format',
     b'exp-revlogv2.2',