TYPO3 Core

problem:
FE rendering is slowed down because many classes call methods of the timetrack object ($GLOBALS['TT']) - even when no BE user is logged in which could see the logged data in the adminpanel.

solution:
check if a be_user is logged in and use the timetrack object only when this is the case. Then wrap each call to a method of $GLOBALS['TT'] in a condition which checks if the global var $USE_TIMETRACK is set.

that works great and results in cached pages rendered 5% faster (see benchmarks below)

info:
the tt object itself is still initialized forbackwards compatibility: it could be needed by extensions which expect it to be initialized

benchmark results:
siege -c 15 -i -b -t 2M -f urls43_perf.txt

trunk
-------------------------------------------------------------
Transactions: 32354 hits
Availability: 100.00 %
Elapsed time: 120.46 secs
Data transferred: 790.16 MB
Response time: 0.06 secs
Transaction rate: 268.59 trans/sec
Throughput: 6.56 MB/sec
Concurrency: 14.97
Successful transactions: 32354
Failed transactions: 0
Longest transaction: 0.53
Shortest transaction: 0.00

patched
-------------------------------------------------------------
Transactions: 34167 hits
Availability: 100.00 %
Elapsed time: 120.48 secs
Data transferred: 834.85 MB
Response time: 0.05 secs
Transaction rate: 283.59 trans/sec
Throughput: 6.93 MB/sec
Concurrency: 14.97
Successful transactions: 34167
Failed transactions: 0
Longest transaction: 0.41
Shortest transaction: 0.00

trunk = 100%
patched = 105.5%

(issue imported from #M11535)