Optimized FlowTransport's use of XXHash

[sfc-gh-satherton]

Optimizations in FlowTransport's use of XXHash to enable inlining, avoid a malloc() when using the stream API, and to avoid the stream API when a message being sent is contiguous within a single packet buffer block.

This is purely a CPU overhead optimization and could be cherry-picked to release-7.0.

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[foundationdb-ci]

AWS CodeBuild CI Report for macOS Catalina 10.15

• CodeBuild project: foundationdb-pr-macos
• Commit ID: a66ed5b
• Result: SUCCEEDED
• Error: N/A
• Build Logs (available for 30 days)

[foundationdb-ci]

AWS CodeBuild CI Report for Linux CentOS 7

• CodeBuild project: foundationdb-pr
• Commit ID: a66ed5b
• Result: FAILED
• Error: Error while executing command: if python3 -m joshua.joshua list --stopped | grep ${ENSEMBLE_ID} | grep -q 'pass=10[0-9][0-9][0-9]'; then echo PASS; else echo FAIL && exit 1; fi. Reason: exit status 1
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[foundationdb-ci]

AWS CodeBuild CI Report for Linux CentOS 7

• CodeBuild project: foundationdb-pr
• Commit ID: bb85ac3
• Result: SUCCEEDED
• Error: N/A
• Build Logs (available for 30 days)