faa76d1b7 qa: Fix inv race in example_test (MarcoFalke)
Pull request description:
There have been intermittent test failures on this script.
```py
File "./test/functional/example_test.py", line 216, in run_test
assert_equal(block, 1)
AssertionError: not(2 == 1)
```
Probably the simplest way to fix them is overriding the `on_inv` method, so that no "colliding" getdata for the blocks are sent out.
Additionally, all getdata are now sent in a single message.
Tree-SHA512: 809c2bbfa90a67fc97905769fcbe90ba9abe1aac1f145530934f56a364835973b94d3302b6be68f4f2987acf333bce146bcc4c878c283301871ba5bb1a9bedb6