Fixes to GlobAsyncRef

[devreal]

1. Use blocking get to retrieve the value.
2. Use non-blocking put to write values.
3. Cache values from get and put.
4. Add flush method to synchronize cached and global value.

Note that a non-blocking get does not make sense but a non-blocking put is perfectly fine. The result of the blocking get will be cached and another get will only be issued after a flush

Fixes #357.

[fuchsto]

Thanks!