posix: bump up telemetry rx/tx buffer sizes to work around deadlock

pios_tcp.c performs all transmits synchronously in the tx_start
handler.  This can lead to deadlocks on the telemetry connection->lock.

Rx thread takes connection lock, calls tx path in response to a get
request.  Blocks on full tx queue.  Tx thread blocks on connection
lock.  Deadlock.

This change does not actually fix the issue.  It simply works around
this deadlock by making the tx buffers larger.