In case of fully populated SSL session cache with no memory left for
new allocations, ngx_ssl_new_session() will try to expire the oldest
non-expired session and retry, but only in case when slab allocation
fails for "cached_sess", not when slab allocation fails for either
"sess_id" or "id", which can happen for number of reasons and results
in new session not being cached.

Patch fixes this by adding retry logic to "sess_id" & "id" allocations.

Patch by Piotr Sikora.

Missing calls to ngx_handle_write_event() and ngx_handle_read_event()
resulted in a CPU hog during SSL handshake if an level-triggered event
method (e.g. select) was used.

In some rare cases this can result in a more smooth sending rate.

According to documentation, calling SSL_write() with num=0 bytes to be sent
results in undefined behavior.

We don't currently call ngx_ssl_send_chain() with empty chain and buffer.
This check handles the case of a chain with total data size that is
a multiple of NGX_SSL_BUFSIZE, and with the special buffer at the end.

In practice such cases resulted in premature connection close and critical
error "SSL_write() failed (SSL:)" in the error log.

No functional changes.

Previously, if SSL buffer was not sent we lost information that the data
must be flushed.

There is no need to flush next chunk of data if it does not contain a buffer
with the flush or last_buf flags set.

A POLLERR signalled by poll() without POLLIN/POLLOUT, as seen on
Linux, would generate both read and write events, but there's no
write event handler for resolver events.  A fix is to only call
event handler of an active event.

The patch saves one EC_KEY_generate_key() call per server{} block by
informing OpenSSL about SSL_OP_SINGLE_ECDH_USE we are going to use before
the SSL_CTX_set_tmp_ecdh() call.

For a configuration file with 10k simple server{} blocks with SSL enabled
this change reduces startup time from 18s to 5s on a slow test box here.