Decrease CPU usage caused by exmpp_tls:send with large data.

Sending one large chunk of data with exmpp_tls:send eats lots of
CPU power and blocks whole Erlang emulator. This is caused by the
fact that encrypted output is read from memory BIO in 1k chunks.
Memory BIO, after reading data, shifts the remaining part.
If large chunks of data (few MB) is sent and then read in 1k
chunks, then a _lot_ of shifting is performed eating CPU.

The solution is to simply allocate binary of the needed size
(amount of data in memory BIO can be retrieved with
BIO_ctrl_pending) and then issue only one read that reads the
whole data.

c_src/exmpp_tls_openssl.c

@@ -387,21 +387,11 @@ exmpp_tls_openssl_control(ErlDrvData drv_data, unsigned int command,
 		break;
 	case COMMAND_GET_ENCRYPTED_OUTPUT:
 		/* Allocate binary to copy encrypted data. */
-		size = BUF_SIZE + 1;
-		rlen = 1;
+		size = BIO_ctrl_pending(edd->bio_write) + 1;
 		b = driver_alloc_binary(size);
 		b->orig_bytes[0] = RET_OK;
 
-		/* Copy data. */
-		while ((ret = BIO_read(edd->bio_write,
-		    b->orig_bytes + rlen, BUF_SIZE)) > 0) {
-			rlen += ret;
-			size += BUF_SIZE;
-			b = driver_realloc_binary(b, size);
-		}
-
-		size = rlen;
-		b = driver_realloc_binary(b, size);
+		BIO_read(edd->bio_write, b->orig_bytes + 1, size - 1);
 
 		break;
 	case COMMAND_GET_PEER_CERTIFICATE: