Implement JdbcBlob.getBinaryStream() and JdbcClob.getCharacterStream() with pos and length arguments

found by UCDetector

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

When using CHAR(N) in PostgreSQL any inserted value smaller than N
characters will be right-padded with spaces to fill all N characters.
See: https://www.postgresql.org/docs/9.1/static/datatype-character.html

This change adds an additional Mode.padFixedStrings and set it to true
in the PostgreSQL Mode. Any value converted to a ValueStringFixed at
INSERT will be right-padded with spaces.

For additional infos see the H2 Google Groups post:
https://groups.google.com/forum/#!topic/h2-database/Luaxdrl137Y

References to BLOB and CLOB objects now have a timeout. The configuration setting is LOB_TIMEOUT (default 5 minutes). This should avoid growing the database file if there are many queries that return BLOB or CLOB objects, and the database is not closed for a longer time.

Some CLOB and BLOB values could no longer be read when the original row was removed (even when using the MVCC mode).

Store CLOBs and BLOBs in maps (StreamStore) to improve performance and avoid using a second connection. (WIP)

inline the "create temporary LOBs" methods into the constructor so that I can make more fields final

re-organise LobStorageBackend code a little, so we don't need to correct the precision of a CLOB later on

Simplify the code and only store DataHandler.