Cleanups to RowDef and associates

Motivating problem was the deadlock between MemoryTableStatus#rowCount() and any DDL inside SchemaManager processing. Two parts to that fix: tighten synchronization in the rowCount() method and add Session as an argument so the "dirty" AIS doesn't have to be used.

As the "dirty" AIS was the real culprit, use this as an excuse to clean up some of the RowDef and gang. In particular, RowDefCache hasn't been a cache for a while so rename to RowDefBuilder. In the area, RowDef, RowData and FieldDef had many completely unused or superfluous members and methods so get rid of them all.

The new test exhibited two failures: the deadlock, if "lucky", and the still problematic RowType issue. Disabled for now due to the latter.