Update synonym implementation

Issue#560 thread leak

Check for unsupported column types in PostgreSQL mode

* 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

eliminate memory estimations for "mem:" case

Add entries for Generic cloud and datawarehouse databases.

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.

Issue#549 Removed UNION ALL requirements for CTE

Table synonym support

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

This reverts commit a45edd78.

and add  a another test case for datediff

Issue#458 TIMESTAMPDIFF YEAR fix

Rollback and delayed meta save.

"Unique index or primary key violation" in TestMvccMultiThreaded

Fixes h2database/h2database#458 TIMESTAMPDIFF()

by using reader-writer lock, reported issue #538

Fix for Issue #543

*Fixes the missing file tree.js in the web console