rpc: provide timestamps in getnep5transfers

[fyrchik]

Set default value for the first timestamp to a week ago.

[codecov]

Codecov Report

Merging #1284 into master-2.x will increase coverage by 0.01%.
The diff coverage is 87.50%.

@@              Coverage Diff               @@
##           master-2.x    #1284      +/-   ##
==============================================
+ Coverage       67.72%   67.73%   +0.01%     
==============================================
  Files             160      160              
  Lines           15877    15892      +15     
==============================================
+ Hits            10753    10765      +12     
- Misses           4585     4587       +2     
- Partials          539      540       +1     

Impacted Files	Coverage Δ
pkg/rpc/server/server.go	84.05% <87.50%> (+0.04%)	⬆️
pkg/core/mpt/trie.go	92.43% <0.00%> (-0.43%)	⬇️

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