Added LastActiveSpanTracker

[jamescrosswell]

#skip-changelog

This is an alternative approach to finding the last active span... rather than a for loop over all the Spans (which we store in a ConcurrentBag), this approach keeps all the spans on a Stack and inspects the most recent one(s) to return the last active span when required... all of which is guarded by a Mutex.

This approach actually uses slightly more memory in the trivial case of <= 10 spans... but vs the loop approach memory consumption for the 100 spans scenario drops by 63.25% from 724.02 KB to 266.09 KB.

Before

Method	SpanCount	Mean	Error	StdDev	Gen0	Gen1	Allocated
'Create spans for scope access'	1	52.05 us	242.80 us	13.309 us	5.2490	1.6479	18.92 KB
'Create spans for scope access'	10	164.61 us	55.00 us	3.015 us	12.2070	4.1504	63.88 KB
'Create spans for scope access'	100	2,287.86 us	1,098.36 us	60.205 us	183.5938	50.7813	724.02 KB

After

Method	SpanCount	Mean	Error	StdDev	Gen0	Gen1	Allocated
'Create spans for scope access'	1	28.45 us	13.270 us	0.727 us	4.6387	1.5869	16.64 KB
'Create spans for scope access'	10	84.98 us	6.361 us	0.349 us	9.3994	2.5635	39.17 KB
'Create spans for scope access'	100	657.54 us	54.581 us	2.992 us	57.6172	16.6016	266.09 KB