contracts: update root cache upon renewal

[peterjan]

This PR updates the root cache in the ContractManager when a contract gets renewed, essentially copying over the roots from existing to renewal. I'm keeping this as a DRAFT @n8maninger because it's merely to illustrate the problem, the real fix is (likely) something else entirely and this will also need a thorough regression test. I don't mind taking a stab at it but I felt this is probably something that you will want to write the fix for yourself.

Context:
In renterd we have a test called TestUploadDownloadSpending that fails locally with AppendSector: proof verification failed on the branch pj/subscription-api. The reason it fails is because hostd builds the proof of a recently renewed contract without sector roots. I sprinkled some logging in the host's contract manager and also the proof code in core and found that we seem to update the database just fine on renewal but the next time the contract updater fetches the roots from the store it returns 0 roots. Causing the host to try and build a proof where secRoots is empty causing it to fail when verifying. I considered the possibility that something was removing those roots but I added panics in all delete and trim methods and those aren't hit during the test.

DEBUG PJ: host: contract manager: store renew contract fcid:d0ddaa094b8082ec760af191a241403846b89136ee1b6343f42bda1044a4e434 -> fcid:9fe0027ea0f4e28861fffcc86a06a93969e49acf98fa18d60de7919f89293c7e
DEBUG PJ: store: renew fcid:d0ddaa094b8082ec760af191a241403846b89136ee1b6343f42bda1044a4e434 -> fcid:9fe0027ea0f4e28861fffcc86a06a93969e49acf98fa18d60de7919f89293c7e updated 3 roots
DEBUG PJ: host: contract updater:  fcid:9fe0027ea0f4e28861fffcc86a06a93969e49acf98fa18d60de7919f89293c7e append sector, new sector roots cnt 1
DEBUG PJ: host: contract updater: fcid:9fe0027ea0f4e28861fffcc86a06a93969e49acf98fa18d60de7919f89293c7e current sector roots cnt 1
DEBUG PJ: core0.2.3: BuildDiffProof
actions: [{Type:Append A:0 B:0 Data:[]}]
secRoots: []
treeHas: []
leafHas: []
DEBUG PJ: core0.2.3: VerifyDiffProof
actions: [{Type:Append A:0 B:0 Data:[]}]
numleav: 3
treeHas: []
leafHas: []
oldRoot: h:5927721564a242691a0ca9d58f810087ca838fe5205989417296cc6181939067
newRoot: h:c076a802cc2791cb30a710a1977e88e540abff86d091014117ed8e0321c43a66
appRoot: [h:c076a802cc2791cb30a710a1977e88e540abff86d091014117ed8e0321c43a66]
OK: false

Couple of things I find especially odd/interesting is that it passes on CI, so somehow this is a race condition, and also if we really consider the contract to be empty after renewal, even for a short period of time, this would wreak some serious havoc on production so it's probably somehow not that big of an issue or a really small window or something.

[n8maninger]

This seems to be an issue with the test, not with hostd