rewrite synchronisation in oaisim/channel simulation

The idea is as follows.

oaisim waits for all tasks to be done for a TTI.
To do so it waits for last_rx_timestamp == current_rx_timestamp,
for all UEs and eNBs.

When it's done, it increases current_rx_timestamp by samples_per_tti,
to inform UEs and eNBs that a new TTI is ready.

In the functions [UE|eNB]_trx_read, we wait for a new TTI to be ready,
that is we wait for current_rx_timestamp != last_rx_timestamp.

The UE has also to wait for processing threads to be done.
In the eNB it's not necessary because the processing is all
done in one thread, that is: read subframe, process this subframe,
emit subframe+4, and then back to read subframe.

The code is not clean. We should not use usleep but semaphores.

Also, do_[DL|UL]_sig should not read from rxdata and write to txdata,
but use internal buffers.

The functions [UE|eNB]_trx_read should call do_[DL|UL]_sig and then
copy from the internal buffers of the channel simulator into rxdata.

The functions [UE|eNB]_trx_write should write data passed into the
internal buffers of the channel simulator.