MFV 316898

7613 ms_freetree[4] is only used in syncing context

illumos/illumos-gate@5f145778012b555e084eacc858ead9e1e42bd149
https://github.com/illumos/illumos-gate/commit/5f145778012b555e084eacc858ead9e1e42bd149

https://www.illumos.org/issues/7613

metaslab_t:ms_freetree[TXG_SIZE] is only used in syncing context. We should
replace it with two trees: the freeing tree (ranges that we are freeing this
syncing txg) and the freed tree (ranges which have been freed this txg).

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Alex Reece <alex@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Matthew Ahrens <mahrens@delphix.com>