JIRA-366: While casting VarChar to BigInt make sure the getByte() method is invoked with the appropriate byte offset.

CastFunctionsSrcVarLen.java:
Use in.start as starting index to invoke getBytes() instead of 0. We always end up with the same row when we use 0

TestCastVarCharToBigInt.java, *.json
Added a test case to read varchar and convert it to BigInt.