[vpr][place] ts_nets_to_update now uses size() instead of num_nets

[robluo]

For runtime reasons, we initialize ts_nets_to_update to the number of nets in the netlist and use a variable num_affected_nets to indicate how many entries are valid. You can experiment to see how runtime is affected if we remove this variable and just reserve elements in this vector and use .size().

The runtime comparison is here: https://docs.google.com/spreadsheets/d/16B169bYI39vUT3wa-vwdqTjXjzJsO9QO7mYUpHC_LdI/edit?usp=sharing

Functional statistic such as final wire length remains the same.
Run time also remains the same.