This is a port from OpenZFS to illumos of the following metaslab improvements from Delphix:

475aa97ca Prevent metaslab_sync panic due to spa_final_dirty_txg

eef0f4d84 Keep more metaslabs loaded

f09fda507 Cap metaslab memory usage

c81f1790e Metaslab max_size should be persisted while unloaded

fe0ea8481 Don't activate metaslabs with weight 0

679b0f2ab Concurrent small allocation defeats large allocation

zfs test run