Specify Default Transport Desired Behavior

[rhcarvalho]

Maybe as part of SDK Features.

Transports should

• Use TCP keep-alive
• Use connection pooling
• Limit number of concurrent calls to Relay (typically and historically 1 request-at-a-time when sending errors; reconsider for transactions and sessions)
• Set client (read/write) timeout
• Report User-Agent sentry-<platform>/<version> or browser-defined as appropriate