Provide a way to call btree_print_tree() to print all BTrees under a given trunk node.

[gapisback]

Existing routine btree_print_tree() works well to print the entire BTree and all its sub-trees from a starting BTree root node. Provide an interface to call this directly from a specific Trunk node.

This will be useful for debugging tree manipulations, especially to see the result of a flush and compact operation.