Rename LeafNodeCodecContext to ValueNodeCodecContext

Rename LeafNodeCodecContext to ValueNodeCodecContext

'LeafNode' prefix has wrong implications, as it is reminiscent
of nodes from 'leaf' statement, when in fact it covers all nodes
considered 'leaf' values in the Binding Object tree. This covers leaf,
leaf-list, anyxml, anydata -- where leaf-list and leaf have actually
differnent handling.

This renames the class to ValueNodeCodecContext, to make a clear
distinction between the two groups.

JIRA: MDSAL-436
Change-Id: I02d60afc1650319efe6cbd90b12eeb47e5152a8f
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>