Switch backend storage to use SQLite

[axllent]

CloverDB (which uses BadgerDB) works really well for Mailpit, however it does come with some massive overheads (for just a small application), and with hindsight just isn't the right choice for Mailpit.

Two reported issues currently affected by this are #8 and #9, and is likely also related to #3.

Comparing CloverDB and SQLite (native Go port) with a persistent database (written to disk) containing 100,000 emails:

	BadgerDB	SQLite
Insert 100 messages	500ms	1,500ms
Memory usage (persistent data)	400-2,500MB	8-20MB
Idle CPU usage	2-3% CPU	0% CPU
Messages index load	9ms	21ms
Single word search	70ms	32ms
Three word search	3,756ms	1,264ms

Note: these are approximate & average values, tested on my laptop (SSD + Intel i7-7600U CPU @ 2.80GHz), and were using actual emails varying in size (ie: not just a simple plain text text email). The values are used simply for a comparison.

Findings

• Data storage speeds reduce quite significantly with SQLite, although I am still able to process and store approximately 75 emails per second (versus 200 per second with CloverDB). SQLite must also use a single query at a time, to very heavy use can result in delayed responses. Both implementations use zstd to compress the raw email in the database to save space.
• The runtime memory requirements are huge by comparison, with SQLite requiring 50-125 times less RAM than CloverDB/BadgerDB at all times.
• BadgerDB appears to use 2-3% CPU when idle (unsure why), SQLite does not suffer from this.
• UI response time is similar for message lists and message loading.
• Single word searches are similar in response time, however SQLite performs much better with multi-word searches.
• The implementation of SQLite also allows Mailpit to perform more defined searching within specific fields, currently limited to:<term>, from:<term>, subject:<term>, and also has:attachment.
• BadgerDB does reclaim some space after deleting enough messages, however it isn't clear exactly how & when this works, and apparently requires the database to be closed and re-opened, plus some additional data written to to the database to activate the pruning. SQLite can run a periodic VACUUM to reclaim space (automatically via Mailpit when needed), which reverts the entire database to less than 40KB when all messages are deleted (Delete all).

Implementation & testing

• Mailpit currently allows either persistent storage, or in-memory storage. Whilst SQLite also has a feature to run in-memory, the chosen approach will be to store "non-persistent" data to a temporary file instead of RAM, which is then automatically deleted on exit. This will mean that memory usage is not directly linked to the number of messages.
• Testing on Windows / Mac needs to be done prior to merging in any code.

Breaking changes

Once implemented, the switch to SQLite will mean that any existing user data is useless, and should be manually deleted to free up space. The database formats are not interchangeable, and once I implement SQLite the original data will no longer be usable. Given the major change, the next release should likely jump a major version, ie: 1.0.0.