StoreUpgrader minor cleanup.

o Remove unused msgLog field and constructor parameter.
o Use FileSystemAbstraction instead of unsafe java.io.File operations.

Conflicts:
	community/kernel/src/main/java/org/neo4j/kernel/impl/storemigration/StoreUpgrader.java
	community/kernel/src/test/java/org/neo4j/kernel/impl/storemigration/StoreUpgraderTestIT.java
Check that original log file exists before trying to copy it.

Conflicts:
	community/kernel/src/main/java/org/neo4j/kernel/impl/storemigration/StoreUpgrader.java
Explictly allow breaking changes to the config API, to clean up all the stuff we made public that is not supposed to be.