init: move asmap code earlier in init process

and update feature_asmap.py and test_runner.py

This commit moves the asmap init.cpp code from the end of "Step 12: start node"
to "Step 6: network initialization" to provide feedback on passing an -asmap
config arg much more quickly. This change speeds up the feature_asmap.py
functional test file from 60 to 5 seconds by accelerating the 2 tests that use
`assert_start_raises_init_error`.

Credit to Wladimir J. van der Laan for the suggestion.

test/functional/feature_asmap.py

@@ -18,8 +18,7 @@ Verify node behaviour and debug log when launching bitcoind in these cases:
 
 6. `bitcoind -asmap` with an empty (unparsable) default asmap file
 
-The tests are order-independent. The slowest tests (missing default asmap and
-empty asmap) are placed last.
+The tests are order-independent.
 
 """
 import os