ovsdb: Correct specification inconsistency between "lock" and "assert".

ovsdb: Correct specification inconsistency between "lock" and "assert".

The "lock" request requires the lock name to be an <id> but it is shown as
<string> in the "assert" operation.  This corrects the "assert"
specification and fixes the suggested naming convention (since ":" is not
valid in an <id>).

This commit also updates the implementation to match the specification.

Reported-by: Jeremy Stribling <strib@nicira.com>