[HBCK2] Add new methods for dealing with missing regions in META while Master is online

Adds new, lightweight option for recovering missing regions in meta, "addFsRegionsMissingInMeta" command, as well "readonly" "reportMissingRegionsInMeta" to show the list of regions to be readded in meta. This is a less intrusive alternative to OfflineMetaRepair, can be used when master and meta are still capable of coming online. Detailed description as provided by the command usage help: {noformat}  addFsRegionsMissingInMeta <NAMESPACE|NAMESPACE:TABLENAME>...    Options:     -d,--force_disable aborts fix for table if disable fails.    To be used in scenarios where some regions may be missing in META,    but there's still a valid 'regioninfo' metadata file on HDFS.    This is a lighter version of 'OfflineMetaRepair tool commonly used for    similar issues on 1.x release line.    This command needs META to be online. For each table name passed as    parameter, it performs a diff between regions available in META,    against existing regions dirs on HDFS. Then, for region dirs with    no matches in META, it reads regioninfo metadata file and    re-creates given region in META. Regions are re-created in 'CLOSED'    state at META table only, but not in Masters' cache, and are not    assigned either. To get these regions online, run HBCK2 'assigns'command    printed at the end of this command results for convenience.    NOTE: If using hbase releases older than 2.3.0, a rolling restart of    HMasters is needed prior to executing the provided 'assigns' command.    An example adding missing regions for tables 'tbl_1' on default    namespace, 'tbl_2' on namespace 'n1' and for all tables from    namespace 'n2':      $ HBCK2 addFsRegionsMissingInMeta default:tbl_1 n1:tbl_2 n2    Returns HBCK2 'assigns' command with all re-inserted regions.    SEE ALSO: reportMissingRegionsInMeta ...  reportMissingRegionsInMeta <NAMESPACE|NAMESPACE:TABLENAME>...    To be used in scenarios where some regions may be missing in META,    but there's still a valid 'regioninfo metadata file on HDFS.    This is a checking only method, designed for reporting purposes and    doesn't perform any fixes, providing a view of which regions (if any)    would get re-added to meta, grouped by respective table/namespace.    To effectively re-add regions in meta, addFsRegionsMissingInMeta should be executed.    This command needs META to be online. For each namespace/table passed    as parameter, it performs a diff between regions available in META,    against existing regions dirs on HDFS. Region dirs with no matches    are printed grouped under its related table name. Tables with no    missing regions will show a 'no missing regions' message. If no    namespace or table is specified, it will verify all existing regions.    It accepts a combination of multiple namespace and tables. Table names    should include the namespace portion, even for tables in the default    namespace, otherwise it will assume as a namespace value.    An example triggering missing regions report for tables 'table_1'    and 'table_2', under default namespace:      $ HBCK2 reportMissingRegionsInMeta default:table_1 default:table_2    An example triggering missing regions report for table 'table_1'    under default namespace, and for all tables from namespace 'ns1':      $ HBCK2 reportMissingRegionsInMeta default:table_1 ns1    Returns list of missing regions for each table passed as parameter, or    for each table on namespaces specified as parameter. ... {noformat}