Support 'ALTER TABLE REBALANCE' MVP

[whitehawk]

Support 'ALTER TABLE REBALANCE' MVP

Problem description:
There were no means to provide segments shrink feature to the 'gprebalance'
tool.

Fix:
Add new command 'ALTER TABLE <table_name> REBALANCE' (MVP level). Details:

1. 'ALTER TABLE <table_name> REBALANCE' supports an optional parameter - target
   number of segments (ex. 'ALTER TABLE <table_name> REBALANCE 2;').
2. If the target number of segments is more than the number of segments in the
   table's distribution policy, rebalance command will invoke the existing
   functionality of 'ALTER TABLE <table_name> EXPAND TABLE' (meaning that expand
   will always be done to the current number of segments in the cluster, even if
   we specified less)
3. If the target number of segments is less than the number of segments in the
   table's distribution policy, the table will be shrunk into the target number
   of segments. For hashed or randomly distributed tables, data from the excessive
   segments is inserted into the target segments, and then for all table types the
   distribution policy is updated for the target number of segments. Data from the
   excessive segments is not removed (we do not want to spend time on it, as most
   likely they will be excluded from the cluster soon anyway).
4. New GUC 'gp_target_numsegments' is added. If the target number of segments is
   not specified for the 'ALTER TABLE <table_name> REBALANCE' command, value of
   'gp_target_numsegments' is used.
5. If 'gp_target_numsegments' is set, all new tables are created using this
   number of segments.

[red1452]

Should be command autocompleted?

postgres=# alter table test 
ADD                  ATTACH PARTITION     DETACH PARTITION     DROP                 INHERIT              OWNER TO             REPLICA IDENTITY     SET                  
ALTER                CLUSTER ON           DISABLE              ENABLE               NO INHERIT           RENAME               RESET                VALIDATE CONSTRAINT  
postgres=# alter table test re
rename            replica identity  reset

[whitehawk]

Should be command autocompleted?

postgres=# alter table test 
ADD                  ATTACH PARTITION     DETACH PARTITION     DROP                 INHERIT              OWNER TO             REPLICA IDENTITY     SET                  
ALTER                CLUSTER ON           DISABLE              ENABLE               NO INHERIT           RENAME               RESET                VALIDATE CONSTRAINT  
postgres=# alter table test re
rename            replica identity  reset

Added autocomplete for this command. It is a psql feature.