CSqlData is const for threadfunctions now to avoid modification from
within the threadfunctions as these might be called several times.
Previously this was a problem as ClearString could possibily be applied
multiple times to the same string.
To solve this the class CSqlString has been added. This class takes a
const char* and copies it. Additionally a clearstring is created from
the given const char*. This enables access to the original as well as
the cleared string safe for sql-statements.
sql_string_helpers got an own source file now.
A crashbug from CSqlServer has been fixed (pointer has not been set
back to 0)
