Remove automatic column addition for empty scan

Originally a column had to be selected to scan a table, so we always added
a cheap column to scan.  When we removed this restriction we forgot to
remove the code that automatically added a column to an empty table scan.
Use aggregation classloader

This fixes a ClassNotFound error when compiling aggregations from a plugin
Refactor ExpressionCompiler

- Remove AbstractFilterAndProject and AbstractScanFilterAndProject and move creation
  of operator factories outside of ExpressionCompiler
- Introduce PageProcessor and CursorProcessor and compile implementations
- Move cursor/page-specific compilers out of ExpressionCompiler and into PageProcessorCompiler/CursorProcessorCompiler
Add JMH benchmark for TPC-H query 6
Process partial pages if PageBuilder becomes full
Buffer bytecode output to avoid tearing