Tenant is not preserved in threads

When using threads, the tenant is not preserved:

```
irb(main):009:0> MultiTenant.current_tenant = 1
=> 1
irb(main):010:0> puts "tenant_id: #{MultiTenant.current_tenant_id}"
irb(main):011:0> thr = Thread.new { puts "tenant_id: #{MultiTenant.current_tenant_id}" }
tenant_id: 1
=> #
tenant_id:
```

Hence queries don't have the tenant clasue and may leak data

```
irb(main):001:0> MultiTenant.current_tenant = 1
=> 1
irb(main):002:0> puts Page.where(id: 999).to_sql
irb(main):003:0> thr = Thread.new { puts Page.where(id: 999).to_sql }
SELECT "pages".* FROM "pages" WHERE "pages"."customer_id" = 1 AND "pages"."id" = 999
=> #
irb(main):004:0> SELECT "pages".* FROM "pages" WHERE "pages"."id" = 999
```

In case this add extra info, we are using concurrent-ruby and we have the same issue

```
irb(main):002:0> MultiTenant.current_tenant = 1
=> 1
irb(main):003:0> thread_pool = Concurrent::FixedThreadPool.new(2)
=>
# end
=> true
tenant_id:
```