Perform CVerifyDB on pcoinsdbview instead of pcoinsTip

Bypassing the main coins cache allows more thorough checking with the same
memory budget.

This has no effect on performance because everything ends up in the child
cache created by VerifyDB itself.

It has bugged me ever since #4675, which effectively reduced the
number of checked blocks to reduce peak memory usage.

• Pass the coinsview to use as argument to VerifyDB

• This also avoids that the first pcoinsTip->Flush() after VerifyDB writes a large slew of unchanged coin records back to the database.