Relation docs updates

See merge request objectbox/objectbox-java!137

Resolve "Test Date property" #169

See merge request objectbox/objectbox-java!139

To enable this, change Transaction.isActive() to not throw if
transaction is closed.

Also, set handle to 0 before calling nativeDelete() to mitigate races

BoxStore: before closing, briefly wait on, then try to close active transactions objectbox/objectbox#1068

See merge request objectbox/objectbox-java!140

They are currently unused, so should be fine.

Also add some API docs.

Otherwise, the native store would wait forever for the write transaction

This reverts commit c3e8630.

isActive() also considered closed flag, which must be ignored here

…#282

Transaction: close write transactions even if store is closing #282

See merge request objectbox/objectbox-java!166

…nReadTx

…#286

Using KTS and a TOML version catalog is the new default for Android
projects, so make it the primary example.

Add all required plugins, add missing repository declaration for plugins
syntax. Add inline instructions.

Also make TOML variant the default, move others to an alternatives
section.