Big refactor of sleuth core API

Instrumentation should be able to get by with only 2
interfaces: TraceManager and TraceAccessor (the former is
not needed if you aren't starting a new Span). No explicit
access to thread locals or manipulation of thread context
is required (except locally where necessary).

A Span is enclosed by a Trace (actually a view of the complete
Trace that would be constructed remotely).

spring-cloud-sleuth-samples/spring-cloud-sleuth-sample-ribbon/src/main/resources/application.yml

@@ -1,6 +1,10 @@
 server:
   port: 3382
 
+sample:
+  zipkin:
+    enabled: false
+
 spring:
   application:
     name: testSleuthRibbon