Fix directory deletion failure in DiskSpaceAllocator

[jsjtzyy]

There was bug in previous refactoring of DiskSpaceAllocator. When
cleaning up old files and directories, code misused file deletion rather
than directory deletion. This PR ensures Utils.deleteFileOrDirectory is
called in this case and extends unit test to mock non-empty directory
deletion.

[codecov-io]

Codecov Report

Merging #1244 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #1244      +/-   ##
============================================
- Coverage      72.3%   72.21%   -0.09%     
+ Complexity     6047     6039       -8     
============================================
  Files           439      439              
  Lines         34910    34909       -1     
  Branches       4430     4429       -1     
============================================
- Hits          25241    25210      -31     
- Misses         8522     8549      +27     
- Partials       1147     1150       +3

Impacted Files	Coverage Δ	Complexity Δ
...ava/com.github.ambry.store/DiskSpaceAllocator.java	90.63% <100%> (+0.71%)	74 <0> (ø)	⬇️
.../com.github.ambry.network/BlockingChannelInfo.java	82.25% <0%> (-6.46%)	16% <0%> (-1%)
...a/com.github.ambry.network/SSLBlockingChannel.java	77.04% <0%> (-4.92%)	4% <0%> (ø)
...java/com.github.ambry.network/SSLTransmission.java	70.22% <0%> (-2.92%)	69% <0%> (-2%)
.../java/com.github.ambry.router/DeleteOperation.java	93.28% <0%> (-1.5%)	44% <0%> (-1%)
...m.github.ambry.replication/ReplicationMetrics.java	94.92% <0%> (-1.21%)	43% <0%> (-1%)
...c/main/java/com.github.ambry.network/Selector.java	79.48% <0%> (-0.86%)	73% <0%> (-1%)
...va/com.github.ambry.replication/ReplicaThread.java	87.33% <0%> (-0.76%)	91% <0%> (-1%)
...rc/main/java/com.github.ambry.store/BlobStore.java	89.58% <0%> (-0.25%)	92% <0%> (-1%)
...ain/java/com.github.ambry.router/PutOperation.java	91.55% <0%> (-0.17%)	113% <0%> (-1%)
... and 2 more

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