[Phase 1] get_custody_chunk_count is broken

[hwwhww]

Issue

In get_crosslink_chunk_count function, it uses attestation.data.latest_crosslink.epoch to calculate crosslink_crosslink_length, but attestation.data.latest_crosslink.epoch field is missing after #874.

[vbuterin]

The best solution I can come up with is to have the number of slots in the crosslink be explicitly listed in the attestation.

(Best to do it as a number of slots, because then we can most easily argue that it's the item length of an SSZ array of flattened block objects, so one could hash together the item length and root to get the SSZ root of the flattened block objects as a dynamic-sized list)