[Radix] Have the radix tree cleanup after itself

Summary: So far, the tree would leak its internals instead of freeing them properly. This patch ensures that the tree will cleanup after itself properly.

Test Plan:

../configure --with-sanitizers=address
make src/test/test_bitcoin
./src/test/test_bitcoin -t radix_tests

Checks that there are no more leaks.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2385