Implement username availability checker

Outlined here: https://github.com/vector-im/riot-web/issues/3605#issuecomment-298679388

```HTTP
GET /_matrix/.../register/available
{
    "username": "desiredlocalpart123"
}
```

If available, the response looks like
```HTTP
HTTP/1.1 200 OK
{
    "available": true
}
```

Otherwise,
```HTTP
HTTP/1.1 429
{
    "errcode": "M_LIMIT_EXCEEDED",
    "error": "Too Many Requests",
    "retry_after_ms": 2000
}
```
or
```HTTP
HTTP/1.1 400
{
    "errcode": "M_USER_IN_USE",
    "error": "User ID already taken."
}

```
or
```HTTP
HTTP/1.1 400
{
    "errcode": "M_INVALID_USERNAME",
    "error": "Some reason for username being invalid"
}
```

Propagate errors sensibly from proxied IS requests

Prefill state caches

Don't fetch state for missing events that we fetched

Add more granular event send metrics

Instantiate DeferredTimedOutError correctly

Call `super` correctly, so that we correctly initialise the `errcode` field.

Fixes https://github.com/matrix-org/synapse/issues/2179.

Speed up filtering of a single event in push

Fix invite state to always include all events

print something legible if synapse already running

web_client_location documentation fix

When creating a new table index in the background, guard against it existing already. Fixes
https://github.com/matrix-org/synapse/issues/2135.

Also, make sure we restore the autocommit flag when we're done, otherwise we
get more failures from other operations later on. Fixes
https://github.com/matrix-org/synapse/issues/1890 (hopefully).

Add some extra logging for edge cases of federation

Check that requested room_id exists

Make state caches cache in ascii