Fix the type of the header receive time

Summary:
This is currently an uint64_t, but:

• It is always assigned int64_t
• The getter returns an int64_t

Let's turn it into a int64_t. Note that this field is not persisted to disk so it doesn't cause issue.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16835