Reader APIs for value versions.

[prataprc]

This issue is applicable only when rdms indexes are
configured to manage older versions for each entries.

Following are the read APIs defined by the Reader trait.

• get()
• iter()
• range()
• reverse()

Above APIs are the faster variant of reading into indexes,
which means, returned Entry may or may-not include all its
previous versions (mutations).

Add a variant of these APIs like:

• get_versions()
• iter_versions()
• range_versions()
• reverse_versions()

where the returned Entry shall always include all available
previous version, but can be slower due to disk-lookup and
merge cost.