importccl: roachtest import/tpch/nodes=8 is unexpectedly slow

[adityamaru]

There is a longstanding TODO above the test definition in registerImportTPCH:

                // TODO(dt): this test seems to have become slower as of 19.2. It previously
		// had 4, 8 and 32 node configurations with comments claiming they ran in in
		// 4-5h for 4 node and 3h for 8 node. As of 19.2, it seems to be timing out
		// -- potentially because 8 secondary indexes is worst-case for direct
		// ingestion and seems to cause a lot of compaction, but further profiling
		// is required to confirm this. Until then, the 4 and 32 node configurations
		// are removed (4 is too slow and 32 is pretty expensive) while 8-node is
		// given a 50% longer timeout (which running by hand suggests should be OK).
		// (07/27/21) The timeout was increased again to 10 hours. The test runs in
		// ~7 hours which causes it to occasionally exceed the previous timeout of 8
		// hours.

We have bumped the timeout to 8 hours in 2019, and subsequently to 10 hours in 2021. The test usually runs in ~7 hours. This issue serves as a placeholder for any investigation into the cause of this slowness. It is likely that the TODO is outdated considering IMPORT has changed considerably since 19.2.

Jira issue: CRDB-8877