CLEANUP: ssl/cli: use a local context for "commit ssl cert"

CLEANUP: ssl/cli: use a local context for "commit ssl cert"

This command only really uses old_ckchs, new_ckchs and next_ckchi
from the appctx's ssl context. The new structure "commit_cert_ctx"
only has these 3 fields, though none could be removed from the shared
ssl context since they're still used by other commands.