Recursively kill all RPC child processes on exit

[shuklaayush]

What I did

Modified the RPC kill() function to recursively kill all child processes. This ensures that hardhat terminates properly on exit.

Also changed timeout of hardhat-fork to 120s to match other forknets.

Related issue: #1199

How to verify it

# Launch brownie. This should launch a new hardhat instance
brownie console --network hardhat
# Terminate brownie. This should kill the active hardhat instance
quit()

# Relaunch brownie. Verify that it launches a new hardhat instance
brownie console --network hardhat

Checklist

• I have confirmed that my PR passes all linting checks
• I have added an entry to the changelog