api: make epoch end height optional

[mitjat]

Fixes an oversight in original API design.

Querying /v1/consensus/epochs -- before (note the first epoch, i.e. the active one):

{"epochs":[{"id":13404,"start_height":8050156,"end_height":0},{"end_height":8050156,"id":13403,"start_height":8049556},{"end_height":8049556,"id":13402,"start_height":8048956}]}

After:

{"epochs":[{"id":13404,"start_height":8050156},{"end_height":8050156,"id":13403,"start_height":8049556},{"end_height":8049556,"id":13402,"start_height":8048956}]}