Add match count to user prompt in DB commands

[bouthilx]

We running orion db rm/set, there is a prompt for the user to confirm the operation, but there is no information about what will be executed (how many trials or exp will be affected?). These stats are printed after the operation but not before (in the prompt). The prompt messages (like here https://github.com/Epistimio/orion/blob/develop/src/orion/core/cli/db/rm.py#L34) should include this information. A function process_trial_count should be implemented like process_trial_rm to count the matching trials/exp before the operation is executed.

@mgermain