channeldb: copy value from boltdb's Get instead of using it directly
channeldb: copy value from boltdb's Get instead of using it directly

This can cause an intermittent panic otherwise if bbolt remaps itself
via munmap and mmap. From bbolt's documentation:

* Byte slices returned from Bolt are only valid during a transaction.
Once the transaction has been committed or rolled back then the memory
they point to can be reused by a new page or can be unmapped from
virtual memory and you'll see an unexpected fault address panic when
accessing it.

aakselrod/lnd,aakselrod/lnd