Bug 19532: Make recalls.status an ENUM

Bug 19532: Make recalls.status an ENUM

This patch makes the status attribute an ENUM, setting the default value
as 'requested' as well. The chosen names are easier to read than single
letters. Also, renamed F into fulfilled (this impacts methods names as
well). This is because 'finished' or 'completed' is more a synonym for
old => 1...

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>