Switch to ImmutableMap
assert that totalBytes > freeBytes and totalBytes > 0
ImmutableMaps in constructor, not getters
make InternalClusterInfoService final
just don't set frequency if < 10s
masterFlag -> isMaster
usages and shardSizes cannot be null through normal usage, remove checks
wrap debug in isDebugEnabled() to skip string allocation
test case should extend ElasticsearchTestCase
use ImmutableMap in DiskThresholdDecider tests
Add a random test for DiskUsage object