Add metadata support to CubDB

[zteln]

Add ability to put, delete, and get metadata key-value pairs from the database that are separated from the regular database entries. The metadata table is stored as a keyword in a separate field at each node, but only present at the root node. Support for handling metadata operations in transactions as well.

The main motivation is to be able to store extra metadata information about the database without having to filter it away when reading, e.g. during select.

• put_metadata/3, delete_metadata/2 and get_metadata/3 functions are exposed in the CubDB module.
• The metadata table is moved to the root during splits and merges while underlying nodes get an empty metadata field.