Replicate slow/large RPC calls to HDFS

Config key: hbase.regionserver.slowlog.systable.enabled Default value: false This config can be enabled if hbase.regionserver.slowlog.buffer.enabled is already enabled. While hbase.regionserver.slowlog.buffer.enabled ensures that any slow/large RPC logs with complete details are written to ring buffer available at each RegionServer, hbase.regionserver.slowlog.systable.enabled would ensure that all such logs are also persisted in new system table hbase:slowlog. Operator can scan hbase:slowlog with filters to retrieve specific attribute matching records and this table would be useful to capture historical performance of slowness of RPC calls with detailed analysis. hbase:slowlog consists of single ColumnFamily info. info consists of multiple qualifiers similar to the attributes available to query as part of Admin API: get_slowlog_responses. One example of a row from hbase:slowlog scan result (Attached a sample screenshot in the Jira) :  \x024\xC1\x06X\x81\xF6\xEC column=info:call_details, timestamp=2020-05-16T14:59:58.764Z, value=Scan(org.apache.hadoop.hbase.shaded.protobuf.generated.ClientProtos$ScanRequest)  \x024\xC1\x06X\x81\xF6\xEC column=info:client_address, timestamp=2020-05-16T14:59:58.764Z, value=172.20.10.2:57348  \x024\xC1\x06X\x81\xF6\xEC column=info:method_name, timestamp=2020-05-16T14:59:58.764Z, value=Scan  \x024\xC1\x06X\x81\xF6\xEC column=info:param, timestamp=2020-05-16T14:59:58.764Z, value=region { type: REGION_NAME value: "cluster_test,cccccccc,1589635796466.aa45e1571d533f5ed0bb31cdccaaf9cf." } scan { a                                                              ttribute { name: "_isolationlevel_" value: "\x5C000" } start_row: "cccccccc" time_range { from: 0 to: 9223372036854775807 } max_versions: 1 cache_blocks: true max_result_size: 2                                                              097152 caching: 2147483647 include_stop_row: false } number_of_rows: 2147483647 close_scanner: false client_handles_partials: true client_handles_heartbeats: true track_scan_met                                                              rics: false  \x024\xC1\x06X\x81\xF6\xEC column=info:processing_time, timestamp=2020-05-16T14:59:58.764Z, value=24  \x024\xC1\x06X\x81\xF6\xEC column=info:queue_time, timestamp=2020-05-16T14:59:58.764Z, value=0  \x024\xC1\x06X\x81\xF6\xEC column=info:region_name, timestamp=2020-05-16T14:59:58.764Z, value=cluster_test,cccccccc,1589635796466.aa45e1571d533f5ed0bb31cdccaaf9cf.  \x024\xC1\x06X\x81\xF6\xEC column=info:response_size, timestamp=2020-05-16T14:59:58.764Z, value=211227  \x024\xC1\x06X\x81\xF6\xEC column=info:server_class, timestamp=2020-05-16T14:59:58.764Z, value=HRegionServer  \x024\xC1\x06X\x81\xF6\xEC column=info:start_time, timestamp=2020-05-16T14:59:58.764Z, value=1589640743932  \x024\xC1\x06X\x81\xF6\xEC column=info:type, timestamp=2020-05-16T14:59:58.764Z, value=ALL  \x024\xC1\x06X\x81\xF6\xEC column=info:username, timestamp=2020-05-16T14:59:58.764Z, value=vjasani