Use http keep-alive instead of individual request connections when talking to Vault server

Right now the HashiCorp Vault key management plugin uses individual HTTP connections to send requests to the Vault key server via the curl library. When dealing with a large number of different keys this is

a) slower then necessary due to the connect overhead, including TLS handshake when using https:
b) can use up a lot of client ports in a short time frame, so potentially running out of available ports while these are in TIME_WAIT state before becoming ready or reuse

As Vault supports http keep-alive we should try to re-use existing curl/http connections for followup requests where ever possible.