Resolves#2054
**Context**: branch creation needs to wait for GC to acquire `gc_cs` lock, which prevents creating new timelines during GC. However, because individual timeline GC iteration also requires `compaction_cs` lock, branch creation may also need to wait for compactions of multiple timelines. This results in large latency when creating a new branch, which we advertised as *"instantly"*.
This PR optimizes the latency of branch creation by separating GC into two phases:
1. Collect GC data (branching points, cutoff LSNs, etc)
2. Perform GC for each timeline
The GC bottleneck comes from step 2, which must wait for compaction of multiple timelines. This PR modifies the branch creation and GC functions to allow GC to hold the GC lock only in step 1. As a result, branch creation doesn't need to wait for compaction to finish but only needs to wait for GC data collection step, which is fast.
See https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency
* Previously there was a single concurrency group per each branch.
As the `main` branch got pushed into frequently, very few commits got
tested to the end. It resulted in "broken" `main` branch as there were
no fully successful workflow runs.
Now the `main` branch gets a separate concurrency group for each commit.
* As GitHub Actions syntax does not have the conditional operator, it is
emulated via logical and/or operations. Although undocumented, they
return one of their operands instead of plain true/false.
* Replace 3-space indentation with 2-space indentation while we are here
to be consistent with the rest of the file.
