Add a new play command that accepts a size_t for iterations. This
allows arbritrary iteration on the signal as well as continous
play (max value of iteration). Use the CONTINOUS_PLAY command for
that.
The old play command (using bool) is deprecated but stays in
for backward capabilities.
The new API calls get_{record,play}_async_metadata() calls are now
available in Python. To look more Pythonic, we change the call signature
and return value to either return `None` or the value (if available).
For comparison, this is the C++ code:
```cpp
uhd::rx_metadata_t md;
if (replay_ctrl->get_record_async_metadata(md, 0.1)) {
cout << "Received metadata! Error code: " << md.strerror() << endl;
} else {
cout << "No metadata received!" << endl;
}
```
In Python, this has the more Pythonic form:
```python
md = replay_ctrl.get_record_async_metadata(0.1);
if md is not None:
print("Received metadata! Error code: ", md.strerror())
else:
print("No metadata received!")
```
