OpenZFS 7613 - ms_freetree[4] is only used in syncing context

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).

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Alex Reece <alex@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Tim Chase <tim@chase2k.com>

OpenZFS-issue: https://www.illumos.org/issues/7613
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/a8698da2
Closes #5598