Minor refactoring for ServerDBInfo.actor.h and StorageServer.actor.h

[xis19]

Put description here...

Code-Reviewer Section

The general guidelines can be found here.

Please check each of the following things and check all boxes before accepting a PR.

• [ ] The PR has a description, explaining both the problem and the solution.
• [ ] The description mentions which forms of testing were done and the testing seems reasonable.
• [ ] Every function/class/actor that was touched is reasonably well documented.

For Release-Branches

If this PR is made against a release-branch, please also check the following:

• [ ] This change/bugfix is a cherry-pick from the next younger branch (younger release-branch or master if this is the youngest branch)
• [ ] There is a good reason why this PR needs to go into a release branch and this reason is documented (either in the description above or in a linked GitHub issue)

[foundationdb-ci]

AWS CodeBuild CI Report for macOS Catalina 10.15

• CodeBuild project: foundationdb-pr-macos
• Commit ID: 45640fe1751199816abdd6ff259f6f0947cb941d
• 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: 45640fe1751199816abdd6ff259f6f0947cb941d
• Result: SUCCEEDED
• Error: N/A
• Build Logs (available for 30 days)