pool: Follow up cd0c482af5338249c064f0cbe889bdc2c1790c0f - Make rep ls output more efficient

This is a follow up patch to https://github.com/dCache/dcache/commit/cd0c482af5338249c064f0cbe889bdc2c1790c0f
The initial commit message did not say that commit cd0c482af5338249c064f0cbe889bdc2c1790c0f actually prevents
rep ls from getting interrupted by a FileNotFoundException.

This patch just improves the data structure and resourcces needs.

Changed some internal data structure to reduce resource needs.

Ticket: 8203
Acked-by: Gerd (verbally)
Target: master, 2.6, 2.7
Require-book: no
Require-notes: no
Patch: http://rb.dcache.org/r/<id>/