new code lens codeWatcher is created for every edit without disposing the previous one

[amunger]

Found while adding some logging to troubleshoot a test, but this could potentially be leaking memory:

Info 13:04:13: Adding hash for 5 = 530ca6997d93 with 2 lines
Info 13:04:13: Cell 8 executed with state Success
Info 13:04:13: updating code lenses for version 2 of document file:///c%3A/src/test/2/script2.py

edit performed
Info 13:04:23: creating new code watcher with matching document file:///c%3A/src/test/2/script2.py
Info 13:04:23: Generating new code lenses for version 3 of document file:///c%3A/src/test/2/script2.py

Info 13:04:26: Adding hash for 6 = 6794747d16e6 with 3 lines
Info 13:04:26: Cell 9 executed with state Success
Info 13:04:26: updating code lenses for version 3 of document file:///c%3A/src/test/2/script2.py
Info 13:04:26: updating code lenses for version 3 of document file:///c%3A/src/test/2/script2.py

edit performed
Info 13:04:31: creating new code watcher with matching document file:///c%3A/src/test/2/script2.py
Info 13:04:31: Generating new code lenses for version 4 of document file:///c%3A/src/test/2/script2.py
edit performed
Info 13:04:33: creating new code watcher with matching document file:///c%3A/src/test/2/script2.py
Info 13:04:33: Generating new code lenses for version 5 of document file:///c%3A/src/test/2/script2.py

Info 13:04:35: Adding hash for 7 = 5da380be1d28 with 5 lines
Info 13:04:35: Cell 10 executed with state Success
Info 13:04:35: updating code lenses for version 5 of document file:///c%3A/src/test/2/script2.py
Info 13:04:35: updating code lenses for version 5 of document file:///c%3A/src/test/2/script2.py
Info 13:04:35: updating code lenses for version 5 of document file:///c%3A/src/test/2/script2.py
Info 13:04:35: updating code lenses for version 5 of document file:///c%3A/src/test/2/script2.py