Don't create noop scopes for akka messaging unless necessary

[bantonsson]

This change tries to minimize the work done when there is no span travelling across an akka message send.

Here are some numbers for an akka message sending benchmark that don't have any spans active.

Version	Average time (s)	stdev	overhead %
none	1.676	0.132	0.000
0.75.0	4.163	0.588	148.426
0.76.0	4.675	0.845	178.973
0.76.1	4.051	0.473	141.741
0.77.0	11.723	3.888	599.550
0.78.1	9.756	3.390	482.148
0.78.2	7.253	0.811	332.792
0.78.3	4.276	0.364	155.163
0.79.0	4.147	0.381	147.496
this PR	1.980	0.218	18.169