t3lib_lock can be cleaned up a bit: The constructor currently returns values
which is impossible, one error case is turned into an exception.
The constructor is additionally covered with unit tests.

Change-Id: I86b2d7e2c1329cefd6a00952c9a024fd926d5abf
Resolves: #29132
Reviewed-on: http://review.typo3.org/4444
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog