Process repositories on multiple threads

Use 'rayon' to parallelise the repository processing. Each repository is
processed in a thread in the default 'rayon' pool.

In order to get thread-safe access to the database, I followed some
advice from a Stack Overflow answer by VasiliNovikov
(https://stackoverflow.com/users/1091436/vasilinovikov):
https://stackoverflow.com/questions/62560396/how-to-use-sqlite-via-rusqlite-from-multiple-threads/62560397#62560397

VasiliNovikov recommended creating a database connection pool using
'r2d2_sqlite'. This way we don't have to share a database connection
between threads, but each thread can have its own connection.

This also means we can remove mutable requirements in a bunch of places
involving our `database::Db` type since we're no longer managing the
database connections directly.

0 files changed, 0 insertions, 0 deletions