Move EMPTY_STATS impl to interface singleton

Conflicts:
	phoenix-core/src/main/java/org/apache/phoenix/schema/PTableImpl.java
	phoenix-core/src/main/java/org/apache/phoenix/schema/stat/PTableStats.java
	phoenix-core/src/main/java/org/apache/phoenix/schema/stat/PTableStatsImpl.java
Rename stat package to stats