dcache-webadmin: put limit on entries in alarms table

In order to avoid OOM.

This is hard-coded as effectively the capacity to hold more than 10000 entries in memory should not be required.  One can always adjust the numbers or refine the query in order to retrieve subsets of a larger set.

Testing: Rechecked interactively.

Target: master
Request: 2.7
Request: 2.6
Patch: http://rb.dcache.org/r/6344/
Bug: http://rt.dcache.org/Ticket/Display.html?id=8084
Require-book: no
Require-notes: no
Acked-by: Karsten