Use parse and asserts from http.servlet

the method "assert_params_in_request" does handle dicts and not
requests. A request body has to be parsed to json before this method
can be used

parse_integer and parse_string can take a request and raise errors
in case we have wrong or missing params.
This PR tries to use them more to deduplicate some code and make it
better readable

Make FederationRateLimiter queue requests properly

popitem removes the *most recent* item by default [1]. We want the oldest.

Fixes #3524

[1]: https://docs.python.org/2/library/collections.html#collections.OrderedDict.popitem

Reduce set building in get_entities_changed

This line shows up as about 5% of cpu time on a synchrotron:

    not_known_entities = set(entities) - set(self._entity_to_key)

Presumably the problem here is that _entity_to_key can be largeish, and
building a set for its keys every time this function is called is slow.

Here we rewrite the logic to avoid building so many sets.

Enforce the specified API for report_event

Use stream cache in get_linearized_receipts_for_room

* Use more portable syntax using attrs package.

Newer syntax

    attr.ib(factory=dict)

is just a syntactic sugar for

    attr.ib(default=attr.Factory(dict))

It was introduced in newest version of attrs package (18.1.0)
and doesn't work with older versions.

We should either require minimum version of attrs to be 18.1.0,
or use older (slightly more verbose) syntax.
Requiring newest version is not a good solution because
Linux distributions may have older version of attrs (17.4.0 in Fedora 28),
and requiring to build (and package)
newer version just to use newer syntactic sugar in only one test
is just too much.
It's much better to fix that test to use older syntax.

Signed-off-by: Oleg Girko <ol@infoserver.lv>

This avoids us from uncessarily hitting the database when there has been
no change for the room

Let's try to include time spent in the DB threads in the per-request/block cpu
usage metrics.

Factor out the resource usage tracking out to a separate object, which can be
passed around and copied independently of the logcontext itself.

Add CPU metrics for _fetch_event_list

Newer syntax

    attr.ib(factory=dict)

is just a syntactic sugar for

    attr.ib(default=attr.Factory(dict))

It was introduced in newest version of attrs package (18.1.0)
and doesn't work with older versions.

We should either require minimum version of attrs to be 18.1.0,
or use older (slightly more verbose) syntax.
Requiring newest version is not a good solution because
Linux distributions may have older version of attrs (17.4.0 in Fedora 28),
and requiring to build (and package)
newer version just to use newer syntactic sugar in only one test
is just too much.
It's much better to fix that test to use older syntax.

Signed-off-by: Oleg Girko <ol@infoserver.lv>

add a Measure block on _fetch_event_list, in the hope that we can better
measure CPU usage here.

Run isort on Synapse