required passing the Column down to Value#convertTo
Note that the test code in testscript.sql was pulled from maxenglander's
PR #572

and making them sort-of regular functions makes it easier to allow
overriding them

The charset collator allows sorting strings according to their binary representation in a given charset.

Remove most of the abstract methods in AbstractTable. Instead add abstract resolve()/asTable() method, that return the backing table.

* Close connections in test
* Call updateMeta in CreateSynonym to make sure changes get persisted
* Drop synonym when the backing is table is dropped to fix ticket #556

This patch adds entries for Generic database entries for cloud
and data warehouse databases. This is solely for convenience
for people using the web server as a convenient frontend for
SQL database.

Conventional RDBMS databases moved into the cloud:

 - Azure SQL (Microsoft, based on SQL Server)
 - Redshift (Amazon, based on PostgreSQL 8)

(Note: Redshift can also use a PostgreSQL driver but the
Redshift driver has significantly better performance.)

Cloud-native databases:

 - Hive 1
 - Hive 2

(Note: hive may be run on a vanilla Hadoop cluster, a Cloudera
cluster, or a Hortonworks cluster. The last two products may
offer custom JDBC drivers.)

(Note: this patch does not address Kerberos authentication
or SSL encryption.)

Data warehouse databases:

 - Impala
 - Snowflake
 - Teradata

Impala is part of the Hadoop ecosystem and provided by Cloudera.
Snowflake and Teradata are both commercial products.

This reverts commit ae6da320, reversing
changes made to 8b018f2b.

This reverts commit a45edd78.

and add  a another test case for datediff

by using reader-writer lock, reported issue #538