Fix crashes in QtHelp when too many connections are made

Fixes a crash when Qt Creator was registering a large number of .qch
files on first startup: In this case the limit of 1000 connections
within one session will overflow, resulting in conflicting connection
names and crashes later on.

Remove this limitation by keeping separate counters per connection name,
and also not wrapping around after 1000.

It's not clear where the 1000 connection limit stems from: The offending
lines predate the first git import of Qt 4.

Task-number: QTBUG-36480
Change-Id: If00276652644efa854a75407d00ba01069bde02a
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>
Update changelog for 5.2.1

Change-Id: I16ff3a410898563857b70d467f6ba8599c722ce6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>