stubby: update to version 0.2.4

stubby: update to version 0.2.4

This upstream release adds support for trust_anchors_backoff_time
configuration parameter. UCI support has been added for this.

This commit also includes a number of clean-ups:
    o change START=50 to START=30 in init file
      Starting earlier in the boot means less chance of missing interface
      trigger events. See: https://github.com/openwrt/packages/pull/4675
    o remove unused variables from init file
    o separate local declarations and assignments in init file
    o add defensive quoting in init file
    o use default values for procd respawn in init file
    o make use of {} in variables consistent in init file
    o remove unused variable from init file

Signed-off-by: Jonathan G. Underwood <redacted>