Truncate large strings in DarkConsole

Summary:
Ref T8597. If a page issues a large query (like inserting a blob into file storage), we may try to utf8ize the entire thing. This is slow and pointless.

Instead, truncate tab data after 4096 bytes before sanitizing.

Test Plan: Adjusted limit to 256 bytes, saw long queries get truncated reasonably.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8597

Differential Revision: https://secure.phabricator.com/D13347