Minimize mutex lock time in target allocator when updating collectors

[alexperez52]

Part of Target Allocator changes

Mutex is locked when collectors are being updated.
Instead, there could be a local var that stores the list of collectors which will replace allocator.collectors. The mutex would then only be locked when the change is ready.

Related PR: #354

[jaronoff97]

We've overhauled how this works and this should be much more efficient now, please let me know if you run into more performance problems here.