bugfix: Data tree node not found

[jdneo]

fix #478

[Eskibear]

It looks you are using await-lock to replace previously self-implemented one. So what's the root cause? Is previous one not working?

[jdneo]

await-lock is just to reuse the module that has the same functionality.

The root cause of this bug is that, before this PR, there are different factors causing the explorer to refresh:

1. when reveal a node, it will trigger the explorer to getChildren() until the node is found
2. When server mode changed, the explorer will be refreshed entirely

Then problem happens, because the refresh is not an atomic operation (multiple getChildren() being called from different levels), thus the multiple refresh operation will race with each other, and break the cache.

So in this PR, I add some condition check that prevent those redundant refresh requests being called during activation. Then during the activation, there will be only one reveal operation being called.