(2.6) 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: 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
Committed: 0bd6afa0aabf9877fe8508fbd1d2c0a5b48d1018
Merge pull request #361 from alrossi/fix/2.6/alarms_table_limit

(2.6) dcache-webadmin: put limit on entries in alarms table