fix: spv client cache

[EthanYuan]

issue from

• fixes The spv instance cache not invalidate properly #38

A simple idea for a fix would be to get the latest spv instance over the network and update the cache as soon as a suitable spv client does not exist in the cache.

[jjyr]

1. Please explain which issue this PR addresses? Does the cache not invalidate properly?
2. Can we handle cache invalidation in the load_spv_instance function? We can pass an argument last_accept_height: Option< usize > to indicate the lowest accepted height.

[EthanYuan]

1. Please explain which issue this PR addresses? Does the cache not invalidate properly?

I wrote issue here:

• The spv instance cache not invalidate properly #38

2. Can we handle cache invalidation in the load_spv_instance function? We can pass an argument last_accept_height: Option< usize > to indicate the lowest accepted height.

Good idea. I've re-pushed the commit for review.