Session duration should be reported in seconds

[rhcarvalho]

Durations should be reported in seconds and are optional.

We're currently incorrectly computing and reporting durations in milliseconds.

Two changes required:

1. Do not send duration for JS Browser sessions. The duration is not relevant at the moment as it doesn't represent anything users can relate to (for now it is essentially a page view, not any sort of long operation).
2. For Node, fix duration calculation to report seconds instead of milliseconds and to use the same clock as we use for errors and transactions. See timestampInSeconds.
3. For this PR, revert the above changes to packages/hub/src/session.ts and merge the changes to packages/browser/src/sdk.ts.

Originally posted by @rhcarvalho in #3608 (comment)