find a way to drop all information related to diagnostics on close solution.

[heejaechang]

currently, we use async events fired to let all listener to clean itself up rather than having special logic for solution close.

this cause async events to accumulate when solution is opened/closed repeatedly in quick succession.

think a way to drop all those at once when solution is closed.

[heejaechang]

this new model reduce number of events we fire on solution close. so with v2, this might not an issue anymore.

#10509

[heejaechang]

with v2, I didn't see this being issue anymore. moving out milestone to see whether we get any report from users.

[heejaechang]

we didn't get any issue reported related to this anymore. closing.