Logs page performance improvements

* Refactored logs handler to make performance testing easier
* Replaced read_backwards with a faster version that regexp-matches newlines
  instead of using slow rfind('\n')
* Improved log reading performance by avoiding unnecessary strptime/strftime
  conversions (these functions are slow in Python)
* Removed 'All' button since it causes timeouts for large files
* Other small logs_tab.jsx fixes
* Removed truncate_log parameter -- all logs are now truncated and there's no
  'All' button
* Added 'from' and 'fetch_older' parameters -- this is to make truly
  incremental fetching of older log entries while keeping cache of file cursor
  position
* Added test case for fetch_older functionality

Change-Id: I9f4deec71499103f01dc0916c8e64aa83e06b084
Closes-Bug: #1382489
Merge "Logs page performance improvements"