Fixed simple DoS in message formatting

I've found easy way to DoS attack with not very large text message.
I think the problem in two things: 
1 Replacement pattern `~text~` is not the last step when formatting the message 
2 Recursion `ngx_http_push_stream_str_replace`

Look at `ngx_http_push_stream_format_message`: 
```
    txt = ngx_http_push_stream_str_replace(message_template, &NGX_HTTP_PUSH_STREAM_TOKEN_MESSAGE_ID, char_id, 0, temp_pool);
    txt = ngx_http_push_stream_str_replace(txt, &NGX_HTTP_PUSH_STREAM_TOKEN_MESSAGE_EVENT_ID, event_id, 0, temp_pool);
    txt = ngx_http_push_stream_str_replace(txt, &NGX_HTTP_PUSH_STREAM_TOKEN_MESSAGE_EVENT_TYPE, event_type, 0, temp_pool);
    txt = ngx_http_push_stream_str_replace(txt, &NGX_HTTP_PUSH_STREAM_TOKEN_MESSAGE_CHANNEL, channel_id, 0, temp_pool);
    txt = ngx_http_push_stream_str_replace(txt, &NGX_HTTP_PUSH_STREAM_TOKEN_MESSAGE_TEXT, text, 0, temp_pool);
    txt = ngx_http_push_stream_str_replace(txt, &NGX_HTTP_PUSH_STREAM_TOKEN_MESSAGE_TIME, time, 0, temp_pool);
    txt = ngx_http_push_stream_str_replace(txt, &NGX_HTTP_PUSH_STREAM_TOKEN_MESSAGE_TAG, tag, 0, temp_pool);
```
After replacing the `~text~` is replacing `~time~` and `~tag~`, so if the message containts `~time~` or `~tag~` these two patterns will also be replaced. 
And now look at `ngx_http_push_stream_str_replace`:
```
        u_char *ret = (u_char *) ngx_strnstr(org->data + offset, (char *) find->data, org->len - offset);
        if (ret != NULL) {
            ngx_str_t *tmp = ngx_http_push_stream_create_str(pool, org->len + replace->len - find->len);
            if (tmp == NULL) {
                ngx_log_error(NGX_LOG_ERR, pool->log, 0, "push stream module: unable to allocate memory to apply text replace");
                return NULL;
            }

            off_t offset_found = ret - org->data;
            ngx_memcpy(tmp->data, org->data, offset_found);
            ngx_memcpy(tmp->data + offset_found, replace->data, replace->len);
            ngx_memcpy(tmp->data + offset_found + replace->len, org->data + offset_found + find->len, org->len - offset_found - find->len);

            result = ngx_http_push_stream_str_replace(tmp, find, replace, offset_found + replace->len, pool);
        }
```
For each occurrence of a pattern in a string, allocate memory for result string. So if there will be many occurrences - it takes a decent amount of time and memory.
For example...
Send message:
```
$ ruby -e 'print "~time~"*30000' | http 192.168.1.229/pub id==testchannel
```
And watch in htop:
![2014-10-16-220325](https://cloud.githubusercontent.com/assets/7323495/4668712/4f5b9b4e-5567-11e4-8be2-865d81580b95.png)

Of course, changing the order in `ngx_http_push_stream_format_message` enough to solve the problem, but PR included fixes for both problem.