Relax buildbox-casd timeout

Description

For background, see:

• BuildGrid/buildbox/buildbox-casd#59 (closed)
• https://mail.gnome.org/archives/buildstream-list/2020-March/msg00005.html

Changes proposed in this merge request:

• Increase max amount of time to wait for buildbox-casd to start
• Since timeout has relaxed, ensure that the buildbox-casd process doesn't die while waiting for it to start

This merge request, when approved, will close: #1222 (closed)

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