PatchDB concurrency startup problem

[baconpaul]

Basically as mentioned in #4824 we have a nasty startup semi-race.
It used to result in uncaught exceptions which were of course very bad
but now it makes a splattage of surge errors

here's how

remove the patches db
start two instances of the plugin in logic or reaper or whatnot
shift-click patch browser in both

this basically starts both populating the database

BUT one of them will get a locked / sqlite3_busy (5) message

which will explode them

So we could do a lot of things but I think the trick is

1. BEGIN IMMEDIATE when we start our transaction
2. If we get a '5' then wait. We can also do this with a https://www.sqlite.org/c3ref/busy_timeout.html
3. If we still get a '5' then give up for a bit but don't drain the queue

something like that replacing the TxnGuard class we hav wrapped around the 'go'

[baconpaul]

Oh and this ties to #2359 as well of course.