Add an ability to set the SSL read buffer size

This capability is required for read pipelining. We will only read in as
many records as will fit in the read buffer (and the network can provide
in one go). The bigger the buffer the more records we can process in
parallel.

Reviewed-by: Tim Hudson <tjh@openssl.org>

include/openssl/ssl.h

@@ -1808,6 +1808,9 @@ __owur int SSL_get_ex_data_X509_STORE_CTX_idx(void);
 # define SSL_set_max_pipelines(ssl,m) \
         SSL_ctrl(ssl,SSL_CTRL_SET_MAX_PIPELINES,m,NULL)
 
+void SSL_CTX_set_default_read_buffer_len(SSL_CTX *ctx, size_t len);
+void SSL_set_default_read_buffer_len(SSL *s, size_t len);
+
      /* NB: the keylength is only applicable when is_export is true */
 # ifndef OPENSSL_NO_DH
 void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,