Avoid thread-unsafe use of SimpleDateFormatter

SimpleDateFormatter is not thread safe, yet there are places in which we
use it in multi-threaded environments - or potentially multi-threaded
environments. The standard approach is to use a thread local instance.

This patch converts the places I could find. In a few cases I conservatively
estimated that a formatter could possibly be used from multiple thread and
converted it even when maybe not strictly required.

The alternative would be to use Java 8's new date/time API, but since we
are refusing to use Java 8 we have to solve the problem in other ways.

Target: trunk
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7219/
(cherry picked from commit a86a9763411fbe5f62a337b81108124e524f88b1)

Conflicts:
	modules/dcache/src/main/java/org/dcache/services/billing/text/Indexer.java
Merge pull request #804 from gbehrmann/fix/2.9/rb7219

Avoid thread-unsafe use of SimpleDateFormatter