DATACMNS-1035 - Moved CustomConversions to Spring Data Commons.

Extracted CustomConversions — whose code has largely been duplicated between the MongoDB, Couchbase and Cassandra modules — into Spring Data Commons. Store-specific extensions can now be contributed via a StoreConversions value type that carries both, store-specific default converters as well as a store-specific SimpleTypeHolder to augment the default list of simple types.

Removed SimpleTypeHolders public default constructor in favour of a protected one and a static DEFAULT instance for plain references.

Original pull request: #210.

Author: odrotbohm