Reject overly-taxing ranged-GET requests

RFC 7233 says that servers MAY reject egregious range-GET requests
such as requests with hundreds of ranges, requests with non-ascending
ranges, and so on.

Such requests are fairly hard for Swift to process. Consider a Range
header that asks for the first byte of every 10th MiB in a 4 GiB
object, but in some random order. That'll cause a lot of seeks on the
object server, but the corresponding response body is quite small in
comparison to the workload.

This commit makes Swift reject, with a 416 response, any ranged GET
request with more than fifty ranges, more than three overlapping
ranges, or more than eight non-increasing ranges.

This is a necessary prerequisite for supporting multi-range GETs on
large objects. Otherwise, a malicious user could construct a Range
header with hundreds of byte ranges where each individual byterange
requires the proxy to contact a different object server. If seeking
all over a disk is bad, connecting all over the cluster is way worse.

DocImpact

Change-Id: I4dcedcaae6c3deada06a0223479e611094d57234
Merge "Reject overly-taxing ranged-GET requests"