Bug fix in sdskv_get_packed.

Adds the following functionalities:

• put_packed
• get_packed
• length_packed
• proxy_put_packed

Updates to sdskv-benchmark to enable overriding parameters from the command line.

Added NULL database and corrected C++ API.

Solves some bugs with sdskv_put_multi, and adds complete implementation of C++ interface.

• Some bugs corrected in get_multi
• New C++ interface now available

• REMI is now optional
• Added sdskv_provider_destroy function

• Improvements to many functions

Corrects some bugs and adds the following features:

• Possibility to register custom migration callbacks to call when receiving a database;
• Possibility to compute the size of a database.