Performance improvements in concurrent update code path

[jmacd]

Is your feature request related to a problem? Please describe.

Improve the syncstate code path that locates the Aggregator given the Attribute Set for an instrument. This is currently contentious is a couple of internal Lightstep binaries.

Describe the solution you'd like

1. Replace RWMutex with Mutex
2. Decouple map lock from record initialization, use a sync.Once to initialize
3. Option to ignore fingerprint collisions

Describe alternatives you've considered
n/a