Merge #11012: Make sure to clean up mapBlockSource if we've already seen the block

Summary:
3f8fa7f Make sure to clean up mapBlockSource if we've already seen the block (Cory Fields)

Pull request description:

Otherwise we may leave them dangling.

Credit TheBlueMatt.

Tree-SHA512: 8be77e08ebfc4f5b206d5ee7cfbe87f92c1eb5bc2b412471993658fe210306789aaf0f3d1454c635508a7d8effede2cf5ac144d622b0157b872733d9661d65c3

Backport of Core PR 11012
https://github.com/bitcoin/bitcoin/pull/11012/files

Test Plan:
make check
ninja check
bitcoind # sanity checking

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2891