Add limits when decompressing gzipped write requests

[docmerlin]

Why

It is possible to construct a gzipped request that can expand to a size that exceeds a reasonable limit, resulting in a denial of service by potentially crashing the server.

The following reads the entire contents of the io.Reader into a []byte

influxdb/http/write_handler.go

Line 208 in 8729cd0

data, err := ioutil.ReadAll(in)

How

The amount of data read from the source io.Reader should be limited, which can be achieved by using a LimitReader.

The default for OSS should remain as unlimited. At some future point, influxd will support external configuration (e.g. a .toml file), where it should be available as an option.

The limit should be configurable to the http package such that Cloud 2 services may override the default.