Allow Devices to have an Expire time?

This issue has been migrated from [#2105](https://github.com/matrix-org/synapse/issues/2105).

---

Currently devices seem to linger around forever unless `/delete_devices` is explicitly used. While this doesn't strongly affect mobile client, other clients such as new logins from web or SDK will create a new-device per login which leads to a proliferation of inactive (and often no longer usable devices, certainly as far as e2e is concerned) - it may make sense to allow for these to be pruned automatically or be filtered in some other way.

example e2e session:
```
Using sessionid GwWWFiaY8Hu0Y9Y+2wRjMc4sRzysHclJXYFsA5IQ69U for device @pik:pik-test:EIIWVXQGWK
Using sessionid Yk0x7cSmmWtcogA2ZxwWjELWtI8/ESM9amhD2F1VXig for device @pik:pik-test:EKVCOJJEMU
Using sessionid gAmL4+jShmCpUTJ3e9GZVr2cDXwuynpJU38OfeKURYc for device @pik:pik-test:GYIGDPERPU
Using sessionid DimrciZY2iYRdFCSeN6HSuHl58lntWzUAYacyeoZVbY for device @pik:pik-test:KGEOVQGHMH
Using sessionid CR2Asx9K5w6Dv13wpHcdkiDtyne6eoZaBWpMlA10ye8 for device @pik:pik-test:QMFYSAFEMT
Using sessionid MzJJORv5bD7FSzNRJAh7a2ZKgwa4N6qDS1hP+o0Hm9k for device @pik:pik-test:LOFXNHZVLE
Using sessionid 4Nn/UVxAc5T1/DgGDOff2r6GmTOvN+w/VYHSuCEgNh0 for device @pik:pik-test:XAHAXVMJGQ
Using sessionid TOBrX2CMPOsSV4W/Y+9Iyc3q0RN3k9y9UYlzpkEhWrg for device @pik:pik-test:KLQFAFVDUG
```