LLRB: API Convert LLRB instance to MVCC instance.

[prataprc]

Sometimes users might want to create an initial index of key,value pairs,
without any concurrent reads. Subsequently, once the initial data set is
loaded, index might go live for concurrent reads and writes at which point
MVCC is required.

This demands that llrb package should expose an API to convert LLRB
instance to MVCC instance. So that users can load initial set of data using
LLRB and convert them to MVCC when reads are needed.