STL traffic: Use the sleep+stop logic

+ PPS limit for AWS set to 1.2 Mpps.
+ The logic is very similar to that one in ASTF driver.
 + This helps for testbeds with high duration stretching (e.g. AWS).
 + Difference: No transaction scale, and we deal with floats.
+ Update loss counting to count unsent packets as lost.
 + Also count "unsent" transactions for other transaction types.
 + If nonzero, log the number of unsent packets/transactions.
+ Make STL and ASTF time overhead constant (called delay) configurable.
+ Subtract delay from approximated_duration, also for ASTF.

Change-Id: I6ee6aa6fba4f110ba1636e1b0ff76cac64383e33
Signed-off-by: Vratko Polak <vrpolak@cisco.com>