mbedtls: Optimize memory usage during handshake
All the certificates in the server's chain except the client certificate
are released as soon as possible to save RAM.
They are therefore not available for examination after handshake,
but we don't care: Mongoose frees them immediately anyway
(`MG_SSL_IF_MBEDTLS_FREE_CERTS` is set).
The savings depend on the length of the chain sent by the server, with AWS it's about 7K.
mbedtLS commit for this is https://github.com/cesanta/mbedtls/commit/e3b9e090ee8542daf9e7feb3588d23e7c13f3ec8
CL: mbedtls: Optimize memory usage during handshake
PUBLISHED_FROM=cbd65ab9e6c532925aa45acf95f1083ca53c4f8c
