Introduce the tombstone for RocksDb

[MathieuDutSik]

Motivation

The tombstone problem has emerged as a problem for RocksDb. We therefore introduce the test here for this store. We also do some corrections and simplification to the testing framework.

Proposal

We propose the following changes:

• Add the testing of read keys from the store as test.
• Iterate the reading of the keys from the store to see the evolution of the performance.
• Introduce the test for RocksDb.

Besides, there are a number of changes for the tests:

• Simplification of the MemoryContext::new_for_testing function since the calls always took the same arguments.
• Elimination of requirement for Debug in the error type since it was not used.
• Put the creation of the testing store in the benchmark code.

Test Plan

The CI is the object of those changes.

Release Plan

Not relevant to TestNet / DevNet.

Links

None.