Minor changes to dbwrapper to simplify support for other databases #7927

During my experiment with using LMDB as database I came across these database-agnostic changes to dbwrapper which make it easier to support other databases, or are small common-sense (I think) cleanups at most:

• Pass parent CDBWrapper into CDBBatch and CDBIterator instead of obfuscation key (in the case of lmdb the batch needs to know about the database, to create a write transaction). This also looks cleaner.
• Incrase privacy of methods that are implementation specific by moving them to a namespace: HandleError, GetObfuscateKey.
• Make constructor of CDBIterator private - they rely on implementation details, others have no business instantiating these.
• Remove GetObfuscateKeyHex - It is an unnecessary method as it is used only two times and only internally, and the whole implementation is HexStr(obfuscate_key).
• Remove throw keywords in function signatures - Using throw() specifications in function signatures is not only not required in C++, it is considered deprecated for various reasons. It is not implemented by any of the common C++ compilers. Their usage is also inconsistent with the rest of the source code.

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throw() is still used in two other files on master:

src/support/allocators/secure.h
src/support/allocators/zeroafterfree.h

but it is not for this PR.

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throw() is still used in two other files on master:

src/support/allocators/secure.h src/support/allocators/zeroafterfree.h

Thanks for mentioning, although I think these are of a different class. They use throw() in the more conventional way to signify that a method or function doesn't ever throw. These could be replaced by the noexcept keyword in c++11.