Flush keepalive batches past keepalive limit

[danieljackins]

This PR immediately sends any batches that are over the 64 KB limit when the keepalive option is turned on to help prevent any data loss.

• [ ] I've included a changeset (psst. run yarn changeset. Read about changesets here).

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: e6cba113184d4575a0d55fbe7fcd986939f0d30e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ----------------------- | ----- | | @segment/analytics-next | Minor |

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