Add a traversal method for the radix tree

Summary:
This method applies a callback to each Leaf in the ascending key order.

It is expected to be used for several purposes:

• With compact proof requests to compute the short ids
• With the mempool to fill the getblocktemplate

Test Plan:

ninja check

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11421