perf(db): optimise indices of mail tables

[JohannesGGE]

Fix: #9162

Needs https://github.com/nextcloud/server/pull/43209 to also replace indices.

What should happen:

oc_mail_messages

• add index on structure_analyzed -> mail_messages_strucanalyz_idx
• drop index mail_messages_mailbox_id because mail_messages_mb_id_uid already contains mailbox_id on first position
• make mail_messages_mb_id_uid unique -> mail_messages_mb_id_uid_uidx

oc_mail_classifiers

• add index on created_at -> mail_class_creat_idx

oc_mail_accounts

• add index on provisioning_id -> mail_acc_prov_idx

oc_mail_smime_certificates

• extend mail_smime_certs_uid_idx to cover (user_id,email_address) -> mail_smime_certs_uid_email_idx
• drop mail_smime_certs_id_uid_idx

oc_mail_tags

• drop mail_msg_tags_usr_id_index because mail_msg_tags_usr_lbl_idx already contains user_id on first position

oc_mail_aliases

• add index on account_id -> mail_alias_accid_idx

oc_mail_trusted_senders

• add index on (user_id,email,type) and drop mail_trusted_senders_type -> mail_trusted_senders_idx

oc_mail_coll_addresses

• add index on (user_id, email, display_name) and drop mail_coll_addr_userid_index and mail_coll_addr_email_index -> mail_coll_idx

[JohannesGGE]

I commented out the index creation of the indices to drop.

It would be possible to also comment out the creation of the indices that are replaced and add the new ones in the migration, to have new instances up to date without "occ missingindices". The Migration will have a lot of checks in this case, which can be confusing. What do you think?

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