Avoid blocking if promotables channel is full.
Avoid blocking if promotables channel is full.

In rare situation it is possible to have `promotables` channel full. In such condition, the `Get` function will be blocked because it calls `promote` function. `Get` function being blocked defeats the purpose of fast cache response and hence may impact the application code in unexpected manner.
In this commit, the `promote` function is modified to use non-blocking channel send construct.
karlseguin/ccache