Improved like comparison.

Since %, _ and \ are special characters in LIKE comparison, they must be escaped.
Escape provided argument in memory, or send REPLACE command to the database.

Added missing Oracle string.Compare and various string methods.

TODO: Oracle still behaves differently from .NET due to NULL and "" being same thing.
Special paths for this behavior are required.
TODO: Postgres can use = and <> on few places istead of LIKE comparison.