perf: Backport allocation reduction optimizations from 2.x

[jasonk000]

Backport of 7 performance PRs from the 2.x branch, reducing object allocations on hot paths in the Eureka client during registry fetch/refresh, deserialization, and instance shuffling operations.

• perf: cache AZ to Region lookup #1607
• perf: avoid Optional allocation in EurekaJacksonCodec hot loop #1608
• perf: Jackson Codec allocation improvements #1609
• perf: avoid Optional in Application getInstances #1610
• perf: avoid allocations when checking if Applications is empty #1611
• perf: avoid allocations in populateInstanceCountMap loop #1612
• perf: Optimize Applications refresh internals #1613

Individual commits are preserved from the original PRs. Two commits from #1613 (2af7d67f + 3eaf5aab) are squashed to avoid a transient guava dependency (that was removed in PR's feedback) that conflicts with master's build.gradle.

Verification

Compared net diff of perf changes on 2.x vs this branch; nothing missed. To verify:

git diff 459fcf59..b8ff519 > /tmp/master.patch
git diff cfe16ad2..e17779d4 > /tmp/diff-2x.patch
diff /tmp/master.patch /tmp/diff-2x.patch