Add a 100K-entry test

This adds a test for serialization of a Map of 100K entries,
each of which has the key leaf and a simple constant non-key leaf.
Unlike other tests, this quantifies how well the encoding works
when faced with large maps.

JIRA: CONTROLLER-1919
Change-Id: I306e5175ba595a0967432393766f8317e7205492
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 468373b62341066eda6abcabe21e0e4ce86b4cc3)