MFC r348561: MFV r348548:
9617 too-frequent TXG sync causes excessive write inflation

illumos/illumos-gate@7928f4baf4ab3230557eb6289be68aa7a3003f38

Reviewed by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Andrew Stormont <andyjstormont@gmail.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>