Cache clauses -> row group mapping

Create a shadow table. For `stats`, it'd be `_stats_rowgroups`.

It contains three columns:

- the clause (eg `city = 'Dawson Creek'`)
- the initial estimate, as a bitmap of rowgroups based on stats
- the actual observed rowgroups, as a bitmap

This papers over poorly sorted parquet files, at the cost of some disk
space. It makes interactive queries much more natural -- drilldown style
queries are much faster, as they can leverage work done by previous
queries.

eg 'SELECT * FROM stats WHERE city = 'Dawson Creek' and question_id >= 1935 and question_id <= 1940`
takes ~584ms on first run, but 9ms on subsequent runs.

We only create entries when the estimates don't match the actual
results.

Fixes #6

parquet/parquet_table.h

@@ -6,16 +6,19 @@
 #include "parquet/api/reader.h"
 
 class ParquetTable {
+  std::string file;
+  std::string tableName;
   std::vector<std::string> columnNames;
   std::shared_ptr<parquet::FileMetaData> metadata;
 
 
 public:
-  ParquetTable(std::string file);
+  ParquetTable(std::string file, std::string tableName);
   std::string CreateStatement();
-  std::string file;
   std::string columnName(int idx);
   std::shared_ptr<parquet::FileMetaData> getMetadata();
+  const std::string& getFile();
+  const std::string& getTableName();
 };
 
 #endif