periodic GC for CSI plugins (#7878)

This changeset implements a periodic garbage collection of unused CSI
plugins. Plugins are self-cleaning when the last allocation for a
plugin is stopped, but this feature will cover any missing edge cases
and ensure that upgrades from 0.11.0 and 0.11.1 get any stray plugins
cleaned up.

command/agent/testdata/basic.hcl

@@ -115,6 +115,7 @@ server {
   job_gc_threshold          = "12h"
   eval_gc_threshold         = "12h"
   deployment_gc_threshold   = "12h"
+  csi_plugin_gc_threshold   = "12h"
   heartbeat_grace           = "30s"
   min_heartbeat_ttl         = "33s"
   max_heartbeats_per_second = 11.0

command/agent/testdata/basic.json

@@ -256,6 +256,7 @@
     {
       "authoritative_region": "foobar",
       "bootstrap_expect": 5,
+      "csi_plugin_gc_threshold": "12h",
       "data_dir": "/tmp/data",
       "deployment_gc_threshold": "12h",
       "enabled": true,