BUG-987: ImmutableSet is appropriate here

Change-Id: Idf204d69636af931c1e2cd454528a276947f29d9
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit 3111721d1ad490ab1eb01a92690120889b54ce11)
Signed-off-by: Robert Varga <rovarga@cisco.com>
BUG-987: these collections are not fields

This builder collections are only used in build() and not used
afterwards. Do not store them in the object, such that they can be freed
as soon as possible.

Change-Id: Iffad875b618f4cecbcda424c6f75275046189c90
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit aa084a0443d864a12e120feef2d449b2ead54da5)
Signed-off-by: Robert Varga <rovarga@cisco.com>