Use error_code along side exceptions

[vinniefalco]

nudb should use and offer boost::system::error_codeinterfaces for all functions natively. The existing APIs which throw exceptions should just call the versions which return error codes and rethrow the error as boost::system::system_error (like Asio does).

An open question is how a caller can determine, in cross-platform fashion, the outcome of certain file operations. For example, that the file did not exist.

[vinniefalco]

Exceptions are gone, error codes are in, develop

[vinniefalco]

Merged to master