Fix missing paren in review bot.

Reviewed at https://reviews.apache.org/r/27880/
Move read-modify-write logic out of AttributeStore.Mutable for simpler layering.

Bugs closed: AURORA-913

Reviewed at https://reviews.apache.org/r/27828/
Send an event for host attributes changing rather than maintenance mode changing.

Bugs closed: AURORA-913

Reviewed at https://reviews.apache.org/r/27889/