Simplify TS alloc: move slot assignment

Move into separate functions:
* move timeslot reservation
* move UL timeslot assignment
* move DL timeslot assignment

Change-Id: I64cf78c5cfc78664766f9769dd5cde632dab92b0
Related: OS#2282
diff --git a/tests/tbf/TbfTest.err b/tests/tbf/TbfTest.err
index d046382..c899199 100644
--- a/tests/tbf/TbfTest.err
+++ b/tests/tbf/TbfTest.err
@@ -5921,7 +5921,7 @@
 Rx=4 Tx=3 Sum Rx+Tx=5, Tta=3 Ttb=1, Tra=2 Trb=1, Type=1
 - Selected DL slots: (TS=0)"..ddDd.."(TS=7), single
 Using single slot at TS 4 for DL
-- Reserved DL/UL slots: (TS=0)"....C..."(TS=7)
+- Reserved DL/UL slots: (TS=0)"...DC..."(TS=7)
 - Assigning DL TS 4
 PDCH(TS 4, TRX 0): Attaching TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=NULL EGPRS), 1 TBFs, USFs = 00, TFIs = 00000001.
 TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=NULL EGPRS) Setting Control TS 4