Change count type in the cars model

The count type is changed from uint16 to uint32.  For some performance/stress
tests, it is desirable to issue 1E7 transactions to provide an adequate sample
size.  Prior to this change, it was impossible to issue a million transactions
without either invoking the RPC several times or using count=0 and stopping
based on log messages.  This makes perf testing easier.

Change-Id: Icf125e45bd85e14df6ed5ad91ddad92a8dd2151b
Signed-off-by: Ryan Goulding <ryandgoulding@gmail.com>