[BUGFIX] MySQLi: Allow persistent connections again

For a persistent connection the hostname needs to be
prefixed with "p:". This was already prepared in the
code, however that variable was not yet used.

Change-Id: I01e20fd69239d0772ca9b371af0058316541f2a9
Resolves: #50781
Releases: 6.2, 6.1
Reviewed-on: https://review.typo3.org/22813
Reviewed-by: Philipp Gampe
Tested-by: Stefan Neufeind
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn