Change synchronized on FileSystemContext to avoid deadlock #15768

Job	Run time
modules: (alluxio.client.cli.,!alluxio.client.cli.fs.)	9m 15s
modules: (alluxio.client.cli.fs.**)	9m 15s
modules: (alluxio.client.fs.,!alluxio.client.fs.concurrent.,!alluxio.client.fs.io.**)	9m 18s
modules: (alluxio.client.fs.concurrent.,alluxio.client.fs.io.)	9m 10s
modules: (alluxio.client.,!alluxio.client.fs.,!alluxio.client.cli.,!alluxio.client.fuse.)	6m 23s
modules: (alluxio.job.,alluxio.master.,alluxio.stress.**)	10m 55s
modules: (alluxio.server.ft.,!alluxio.server.ft.journal.raft.,!alluxio.server.ft.journal.ufs...	11m 57s
modules: (alluxio.server.ft.journal.raft.,alluxio.server.ft.journal.ufs.)	11m 52s
modules: (alluxio.server.,!alluxio.server.ft.)	7m 8s
	1h 25m 13s

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