tstore: Freeze trillian trees.

This commit adds episodic checks to the tstore backend that freeze any
trillian trees for records that have been updated to a status that no
longer allows updates, such as censored or archived, and that have a
final dcr timestamp appended onto the tree.

The reason we need this requires some background knowledge on the
trillian architecture.

The trillian_log_signer polls the MySQL database at a fixed interval,
looking for leaves that have been queued up and are waiting to be
appended onto a tree. It does this for all trees that have an ACTIVE
status.

Trillian was designed to be used for a small number of trees that have
infrequent writes, but that can get very large over time. The
recommended log_signer_interval, i.e. the polling interval, was 2-3
seconds.

The way we use trillian is quite different from it's intended use case
and you see this reflected in the performance of trillian on our
servers. We set the log signer interval to 200ms because we require the
leaves be appended onto a tree in order for a write to be considered valid.
We also use a new tree for each record. This results in a large number of 
trees that get polled by the log signer every 200ms and is why the CPUs 
spin on our servers. Moving the status of trees that can no longer be 
modified to FROZEN will help reduce this load.

Bump version to v1.3.1.

ticketvote: Cast vote error format.

This commit changes the ErrorCode field in the CastVoteReply structs
to a pointer, in order to fix a bug where default error code value
ErrorCodeInvalid was returned on success.

After this, in case of success the error code pointer will be nil.

pi: Fix typo in `Summary.Status` field json key.

This diff fixes a typo in the pi API Summary struct where the json key
of Status was defined as "string" instead of "status".

politeiawww: Add login to auth router.

This diff adds the login route to the auth router so that it is CSRF
protected.

politeiawww: Handle client conn drops.

This diff adds in error handling for when the client drops the
connection.

ticketvote: Fix summary best block bug.

This diff fixes a bug that was causing the best block in a vote summary
to not be accurate when the summary was being pulled from the cache.

Make trickler truly random instead of using random bucketized duratio…

…ns (#1368)

* Make trickler truly random instead of using random bucketized durations

* Remove comments now that the code has been validated

gitbe: Set vote duration as a plugin setting.

This diff adds the vote duration min/max as a plugin setting to the
gitbe decred plugin. This allows the min/max to be lowered when on
testnet to make testing easier.

politeiawww already allows this.