lib/repo: Disable locking by default, add locking=true boolean

I want some time to play with this more with different callers and work through
test scenarios. Let's disable the locking by default for now, but make it easy
to enable.

Closes: #1375
Approved by: jlebon

tests/test-concurrency.py

@@ -43,6 +43,7 @@ subprocess.check_call(['ostree', '--repo=repo', 'init', '--mode=bare'])
 # and we don't need xattr coverage for this
 with open('repo/config', 'a') as f:
     f.write('disable-xattrs=true\n')
+    f.write('locking=true\n')
 
 def commit(v):
     tdir='tree{}'.format(v)