WIP Add connection pool option

[HarlemSquirrel]

# Example usage
ldap_with_pool = Net::LDAP.new(
  host: host_name,
  port: 389, 
  connection_pool: { 
    size: 4, # Maximum connections to open
    timeout: 15 # Number of seconds to wait for a connection to free up before raising a timeout error
  }
)

See https://github.com/mperham/connection_pool#usage