esp32: Apply the LWIP active TCP socket limit.

This is a workaround for a bug in ESP-IDF where the configuration setting
for maximum active TCP sockets (PCBs) is not applied.

Fixes cases where a lot of short-lived TCP connections can cause:

- Excessive memory usage (unbounded number of sockets in TIME-WAIT).
- Much higher risk of stalled connections due to repeated port numbers. The
  maximum number of active TCP PCBs is reduced from 16 to 12 to further
  reduce this risk (trade-off against possibility of TIME-WAIT
  Assassination as described in RFC1337).

This is not a watertight fix for the second point: a peer can still reuse a
port number while a previous socket is in TIME-WAIT, and LWIP will reject
that connection (in an RFC compliant way) causing the peer to stall.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

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