BUG-4803: fix MutableOffsetMap's insertion order

When a pairing backed by the array is removed, the order of pairings
would not follow the insertion order. This patch introduces an explicit
removal marker object and when the marker is present we defer to
newKeys map.

Change-Id: Ib27313e0df44754f5219d64126e0ec2a472a503f
Signed-off-by: Robert Varga <rovarga@cisco.com>