fix(plugin): add lock for plugin migrations

src/plugin/lock.rs

src/plugin/migration.rs

src/plugin/mod.rs

test/conftest.py

test/int/test_plugin.py

test/testplug/testplug_w_migration_2/0.1.0/author.db

test/testplug/testplug_w_migration_2/0.1.0/manifest.yaml