9486 reduce memory used by device removal on fragmented pools

In the most fragmented real-world cases, this reduces memory used by the
mapping from ~1GB to ~50MB of RAM per 1TB of storage removed. Less
fragmented cases will typically also see around 50-100MB of RAM per 1TB
of storage.

illumos/illumos-gate@cfd63e1b1bcf7ba4bf72f55ddbd87ce008d2986d

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Tim Chase <tim@chase2k.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>