During IBD, when doing pruning, prune 10% extra to avoid pruning again soon after

Summary:
Pruning forces a chainstate flush, which can defeat the dbcache and harm performance significantly.

Backport of Bitcoin Core PR11658
https://github.com/bitcoin/bitcoin/pull/11658

Test Plan:

make check

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3986