[YSQL] Support backup for pre-split multi-tablet range tables

[ndeodhar]

Jira Link: DB-2111
Currently ysql_dump being used for distributed backups only adds SPLIT INTO clause (or tablet partition information) for hash tables. We should add support for pre-split range tables.

[OlegLoginov]

The idea is simple:

1. CREATE TABLE ... SPLIT AT VALUES... creates the range-table and the split points are stored in the Master table record in the sys-catalog.
2. ysql_dump should get the info and decode back the stored info into the SPLIT AT VALUES () - so ysql_dump must generate correct/original CREATE TABLE statement.

[OlegLoginov]

Example of the range table: https://docs.yugabyte.com/latest/api/ysql/the-sql-language/statements/ddl_create_table/

SPLIT AT VALUES
For range-sharded tables, you can use the SPLIT AT VALUES clause to set split points to presplit range-sharded tables.

Example

CREATE TABLE tbl(
  a int,
  b int,
  primary key(a asc, b desc)
) SPLIT AT VALUES((100), (200), (200, 5));

In the example above, there are three split points and so four tablets will be created:

tablet 1: a=<lowest>, b=<lowest> to a=100, b=<lowest>
tablet 2: a=100, b=<lowest> to a=200, b=<lowest>
tablet 3: a=200, b=<lowest> to a=200, b=5
tablet 4: a=200, b=5 to a=<highest>, b=<highest>

[jaki]

Commits b14485a and 96beb9e add generic YSQL backup/restore handling for any partitioning mismatch. Even if the SPLIT clause provided by ysql_dump (or lack thereof) causes partitioning that doesn't match with what's in the backup, these commits add checks to detect that then repartition the live table to match what's in the backup before/during importing.

In theory, this fix should extend to range partitioned tables. However, since the repartition takes time (and locks on master), it may be worth adding a SPLIT AT VALUES clause in ysql_dump to avoid the repartition upon restore (for most, not all cases, for same reasons as in hash partitioning: see issue #8229).

Also, @yifanguan, note that the first commit I mention adds a disabled test YBBackupTest.TestYSQLRangeSplitConstraint. Please enable it with your changes.

[mheer9]

AS requested schema DDL attached to ticket
https://yugabyte.zendesk.com/agent/tickets/2562

[m-iancu]

Closing as this is solved for backups by 3156825.
However for ysql_dump this is still not solved and should be addressed in a separate issue.