Start to avoid using globally defined namespaces

The basic idea: use "uncommon" namespaces inline instead of globally.

This PR is rather tolerant and does not remove global usings, but I think untangling dependencies etc. would be the next step.

The following are available as well - each of them for a specific namespace, but I felt like this would be an overkill to begin with:

- std: https://github.com/dexX7/bitcoin/tree/mcore-namespace-std
- json_spirit: https://github.com/dexX7/bitcoin/tree/mcore-namespace-json_spirit
- boost: https://github.com/dexX7/bitcoin/tree/mcore-namespace-boost
- leveldb: https://github.com/dexX7/bitcoin/tree/mcore-namespace-leveldb