neutrino: add block cache to config
neutrino: add block cache to config

This commit adds a Block Cache option to the neutrino Config struct so
that a custom block cache can be passed to neutrino. If a block cache is
present in the cache then it is used instead of creating a new one. This
will be useful once a block cache is available in LND that can then be
shared with neutrino.

lightninglabs/neutrino