Change synchronized on FileSystemContext to avoid deadlock #15767

Job	Run time
modules: (alluxio.client.cli.,!alluxio.client.cli.fs.)	9m 47s
modules: (alluxio.client.cli.fs.**)	9m 42s
modules: (alluxio.client.fs.,!alluxio.client.fs.concurrent.,!alluxio.client.fs.io.**)	11m 4s
modules: (alluxio.client.fs.concurrent.,alluxio.client.fs.io.)	9m 44s
modules: (alluxio.client.,!alluxio.client.fs.,!alluxio.client.cli.,!alluxio.client.fuse.)	6m 52s
modules: (alluxio.job.,alluxio.master.,alluxio.stress.**)	10m 54s
modules: (alluxio.server.ft.,!alluxio.server.ft.journal.raft.,!alluxio.server.ft.journal.ufs...	12m 16s
modules: (alluxio.server.ft.journal.raft.,alluxio.server.ft.journal.ufs.)	4m 58s
modules: (alluxio.server.,!alluxio.server.ft.)	7m 49s
	1h 23m 6s

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