Schema: stop using NULL fields

[zeha]

I propose to make these fields NOT NULL:

• domains.master (DEFAULT '')
• domains.account (DEFAULT '')
• domains.last_check (DEFAULT 0)
• domains.notified_serial (DEFAULT 0) -- depending on '0 is not a valid serial' (?)
• domainmetadata.*
• cryptokeys.*
• records.change_date
• records.disabled
• tsigkeys.*

  Other fields in records table

Maybe someone else can shed light on the NULL status of the other fields.

Why?

IMO it'd be a lot better if inside pdns we can depend on fields being filled with some value (even if it's the empty string or 0), AND we can write to those fields the universal empty values (0/'') without writing any database-specific code.

See #2211 and #2064 for cases where we are/should be writing to currently NULLable fields but users (may) expect us to write NULL and we aren't.

[pieterlexis]

see also #3024

[pieterlexis]

this will be a schema change (that we don't want for 4.0.0, moving to 4.1.0)