Tune default threads for magma

As part of MB-39375 KV added a new method of configuring threads for magma (num_storage_threads and magma_flusher_thread_percentage). The default values that scale with the number of writer_threads caused regressions in magma tests (MB-41100). As a short term fix, KV have re-introduced the old default value as a hard coded default in magma-kvstore.cc with a TODO for this MB.

Before release, the magma team must run tests to determine suitable defaults for the number of magma threads as the hard coded value of 20 is likely too many threads for small test setups (docker etc.) and is likely not enough threads for large setups (64core+ machines with multiple disks).