Tweak build files.
Set new descriptor/metrics-lib version.

As part of this, set Java source and target version to 7 in two build
files and add a missing library for running tests.

Implements #21535.

This patch fixes a bug where we'd consider two resource strings with
the same first 2048 characters as two distinct resource strings
internally, but which the database considers the same, because it only
stores the first 2048 characters.  In reality, these are just hacking
attempts or broken clients, so we can as well discard these lines
entirely and not bother any further.

Implements #21236.

Fixes #21223.

This reverts commit 0a6320b6.

atagar points out, and meejah confirms, that txtorcon doesn't parse
most of the descriptors provided by CollecTor, except for
microdescriptors which it parses for its own purposes.  Sounds like
Python developers are better off using Stem for parsing descriptors,
and that's the main use case when they come to this page.

Fixes #21228.

Fixes #21175.

s/The following/This/