MFV r337223:
9580 Add a hash-table on top of nvlist to speed-up operations

illumos/illumos-gate@2ec7644aab2a726a64681fa66c6db8731b160de1

Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Serapheim Dimitropoulos <serapheim@delphix.com>