Commits improving performance counters related to mailing * introduction of message_log (https://github.com/odoo/odoo/commit/8ddca6f95a52d36c4a8731dcf93413e8206dce81) and its application in the mail_thread (https://github.com/odoo/odoo/commit/99be64dc174af9ad7eb739575215aa03260e833b and https://github.com/odoo/odoo/commit/74ec8a95a0f7b1aadd847a728b4d9bc24daca495). This commit has a big impact on the runbot installation query count (110K on 700K, about 15%); * moving templates from mail.templates to qweb (https://github.com/odoo/odoo/commit/2f7593761c1ea432e2e58045150150356816f868). This merge has an impact on the runbot installation query count (10K on 680K); * removing (un)follow buttons in notification emails (https://github.com/odoo/odoo/commit/11d2d30e91758baaa9c6241a86f97ae3dfe52ad2). This commit has an impact on the runbot installation query count (8K on 690K); * removing 'always' tracking option, meaning less tracking values are created when writing on a record (https://github.com/odoo/odoo/commit/c99de4551583e801ecc6669ac456c4f7e2eef1da). No impact is given as it is mixed with another commit that decreases performances. Commits decreasing performance counters related to mailing * adding the discussion subtype consideration in data send to the bus (https://github.com/odoo/odoo/commit/c99de4551583e801ecc6669ac456c4f7e2eef1da). Future improvements will come that improve bus management in chatter and channels. Apart from those commits there are several commits notably from @rco-odoo that improves performances that are not listed here.