fix(server): switch rocksdb backend to memory when executing gremlin example

[VGalaxies]

An error occurs when using Gremlin-Console Stand-alone in offline mode:

Error in scripts/example.groovy at [26: graph.serverStarted(GlobalMasterInfo.master("server-tinkerpop"))] - Failed to update/query server info: java.util.concurrent.ExecutionException: org.apache.hugegraph.backend.BackendException: Table 'hugegraph+v' is not opened

Due to difficulty in pinpointing the cause and considering the above scenario as an example only, the RocksDB backend will be changed to the memory backend.

@imbajin Perhaps the documentation at https://hugegraph.apache.org/docs/clients/gremlin-console/ also needs corresponding adjustments. It is not recommended for users to use it in production environments and is intended for example purposes only.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.09%. Comparing base (1228dec) to head (5c06a6b).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2518      +/-   ##
============================================
+ Coverage     61.03%   61.09%   +0.05%     
  Complexity      827      827              
============================================
  Files           579      579              
  Lines         46232    46232              
  Branches       6275     6275              
============================================
+ Hits          28219    28246      +27     
+ Misses        15198    15174      -24     
+ Partials       2815     2812       -3     

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