Feature: Mirrors

[mhutchinson]

Trillian v1 supported mirrors, where a log could be continually cloned into trillian (via migrillian) and then served from this new location in addition to the original location. This was primarily used to provide mirrors for the CT ecosystem. In mirror mode, the mirror never creates its own checkpoints, and instead serves copies from the origin log.

This feature may be useful in Tessera, and this issue is raised to track the demand signal from the community. It is unclear how much of the core Tessera libraries would truly be exercised by such a feature. It could be that the way to implement this feature barely relies on any of the Tessera code (e.g. sequencing, DBs, antispam, checkpoint signing, etc are all unneeded for this feature).

[mhutchinson]

Looks like @AlCutter is already working on this, so assigning to reflect that.