server: Set max refresh sessions threshold to 8

[yondonfu]

What does this pull request do? Explain your changes. (required)

This PR sets the max refresh sessions threshold to 8.

Prior to this PR, a B would refresh its session list (resulting in running orchestrator discovery to create a new session list) whenever the size of the list falls below numOrchs / 2 where numOrchs is the # of Os that the B is aware of. The problem with this check is illustrated by the following example:

• numOrchs = 100
• Only 50 Os out of 100 respond during discovery
• As a result, the typical size of the session list is 50
• If a single O in that session list returns an error and is suspended from the list the size of the list falls to 49
• B refreshes its session list
• Repeat the above process

The consequence is that B can end up running discovery/refreshing its session list really frequently.

This PR changes B's behavior so that if numOrchs / 2 > 8, then B will only refresh its session list if the size falls below 8. However, if numOrchs / 2 < 8 then B will refresh its session list if the size falls below numOrchs / 2. This means that if B is aware of a lot of Os (i.e. on the public network) it will wait until the size of the session list falls to a lower level before refreshing. But, if B is not aware of many Os (i.e. in a smaller local private network) it will refresh when the session list is at a higher level.

Specific updates (required)

See commit history.

How did you test each of these updates (required)

Tested manually. Would like to add unit tests for this area of the codebase but haven't gotten to it yet...

Does this pull request close any open issues?

Fixes #2080

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[jailuthra]

LGTM