Change JsonRpc exit to use wait->close

[carllin]

Problem

JsonRpcService exit does not currently guarantee threads have finished cleanup

Summary of Changes

1. Change to use wait->close() in jsonrpc exit
2. Change fullnode_exit() (invoked through RPC) to call the exit() method on a validator_exit() object which stores all the closures necessary to shutdown a validator

Fixes #

[codecov]

Codecov Report

Merging #5566 into master will increase coverage by 7%.
The diff coverage is 92.9%.

@@           Coverage Diff            @@
##           master   #5566     +/-   ##
========================================
+ Coverage    70.3%   77.3%     +7%     
========================================
  Files         217     217             
  Lines       45179   41068   -4111     
========================================
- Hits        31774   31773      -1     
+ Misses      13405    9295   -4110