Fix 2 issues related to thread safety in CustomProperties

[guperrot]

• Avoid concurrent modification exceptions while and after calling setCustomProperties by taking a snapshot of properties when we call setCustomProperties.
• Avoid bypassing the size limit by inserting keys from multiple threads.

[guperrot]

It's not the same fix as in EventProperties as we don't do traversals in multiple places for CustomProperties (for EventProperties we traverse also when using inheritance in targets and that would have been harder to use synchronized for that) as opposed to a single copy traversal for snapshot.

Plus we have the size check that is done at set time and needs to be atomic as well which was not achieved by using concurrent map (in event properties, the size check is done later on a snapshot copy and only for AppCenter).

Also concurrent map was requiring to use an object marker for null instead of null so it was introducing more changes.

FYI the alternate version of the fix was cb45279.