Keep more metaslabs loaded

With the other metaslab changes loaded onto a system, we can 
significantly reduce the memory usage of each loaded metaslab and 
unload them on demand if there is memory pressure. However, none 
of those changes actually result in us keeping more metaslabs loaded. 
If we don't keep more metaslabs loaded, we will still have to wait 
for demand-loading to finish when no loaded metaslab can satisfy our 
allocation, which can cause ZIL performance issues. In addition,
performance is traditionally measured by IOs per unit time, while 
unloading is currently done on a txg-count basis. Txgs can take a 
widely varying range of times, from tenths of a second to several 
seconds. This can result in confusing, hard to predict behavior.

This change simply adds a time-based component to metaslab unloading. 
A metaslab will remain loaded for one minute and 8 txgs (by default) 
after it was last used, unless it is evicted due to memory pressure.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
External-issue: DLPX-65016
External-issue: DLPX-65047
Closes #9197

man/man5/zfs-module-parameters.5

@@ -398,7 +398,7 @@ the least recently used metaslab to prevent the system from clogging all of
 its memory with range trees. This tunable sets the percentage of total system
 memory that is the threshold.
 .sp
-Default value: \fB75 percent\fR
+Default value: \fB25 percent\fR
 .RE
 
 .sp
@@ -469,6 +469,34 @@ angular velocity disk drive.
 Use \fB1\fR for yes (default) and \fB0\fR for no.
 .RE
 
+.sp
+.ne 2
+.na
+\fBmetaslab_unload_delay\fR (int)
+.ad
+.RS 12n
+After a metaslab is used, we keep it loaded for this many txgs, to attempt to
+reduce unnecessary reloading. Note that both this many txgs and
+\fBmetaslab_unload_delay_ms\fR milliseconds must pass before unloading will
+occur.
+.sp
+Default value: \fB32\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBmetaslab_unload_delay_ms\fR (int)
+.ad
+.RS 12n
+After a metaslab is used, we keep it loaded for this many milliseconds, to
+attempt to reduce unnecessary reloading. Note that both this many
+milliseconds and \fBmetaslab_unload_delay\fR txgs must pass before unloading
+will occur.
+.sp
+Default value: \fB600000\fR (ten minutes).
+.RE
+
 .sp
 .ne 2
 .na