Jaeger/Zipkin remote endpoint cache improvements

[alanwest]

Zipkin had a remote endpoint cache to reduce allocations:

opentelemetry-dotnet/src/OpenTelemetry.Exporter.Zipkin/Implementation/ZipkinActivityConversionExtensions.cs

Lines 109 to 117 in 465fc09

	#if !NET452
	remoteEndpoint = RemoteEndpointCache.GetOrAdd((hostNameOrIpAddress, attributeEnumerationState.Port), ZipkinEndpoint.Create);
	#else
	var remoteEndpointStr = attributeEnumerationState.Port != default
	? $"{hostNameOrIpAddress}:{attributeEnumerationState.Port}"
	: hostNameOrIpAddress;
	remoteEndpoint = RemoteEndpointCache.GetOrAdd(remoteEndpointStr, ZipkinEndpoint.Create);
	#endif

Jaeger does not have a cache. @CodeBlanch and I discussed (#1195 (comment)) making one.

I've also raised a concern regarding the potential for unbounded memory growth of Zipkin's cache: #1168 (comment)

At a minimum, we should add a cache for Jaeger, but we may also want to consider a cache expiration or size limit for both Jaeger and Zipkin.

[martinjt]

Work has been done here, please reopen if you think there is more to be done however, these exporters are being thought about, separately, as candidates to move into contrib.