Improve circ transaction performance by indexing NULL circs (Launchpad 587031)

Improve circ transaction performance by indexing NULL circs (Launchpad 587031)

Creating a unique index on target_copy in the action.circulation table where
the checkin_time is NULL will increase the speed of circ transactions,
particularly on systems with a lot of traffic. The uniqueness also
ensures that a given item will not be able to be checked out multiple times
concurrently.

Thanks to James Fournie from BC Sitka for suggesting the index in Launchpad
bug 587031!

git-svn-id: svn://svn.open-ils.org/ILS/trunk@16589 dcc99617-32d9-48b4-a31d-7c20da2025e4