Free keys and cert before conf, not after

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Renegotiation is disabled, so we don't need them.
Reduces per-connection usage when idle by 3K.

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 - Only allow one TCP segment in flight. Each segment is 1.5K and it
   adds up. This may increase latency, but memory is precious on ESP.
   At teh same time, optimize use of tcp_output: do not send a new segment
   on every write, only call tcp_output at the end of connection
   processing during poll.

 - Build LWIP with DNS disabled. We have our won resolver, and LWIP's
   allocates large static buffers (1K).

 - mbedTLS: Deallocate peer's certificate at the end of SSL handshake,
   reduces idle SSL connection footprint by ~1.5K.

 Some tweaks to the heap log viewer (which made all the above possible).

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Also performs trnasparent SPIFFS -> SLFS cert copy.

If the file's extension is .pem, convert it to DER format and put on SLFS.

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Also fixed a memory leak on reconnect: previously, each reconnect was
creating a `struct mg_connection` which was never reclaimed

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Useful for debugging at level 3

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The only client of `mg_http_common_url_parse` (namely,
`mg_connect_http_base`) expects `port_i` to be the index in the address
string at which the port was added.

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Needed by TI libc for use with add_device

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If the path starts with /, do not drop it.
There are no directories on SLFS, but use of /pretend/paths/to/files.txt
is common. What we do drop is the ./ prefix (added by mongoose when document_root=.)

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`MG_ENABLE_...` macros are not yet renamed because it involves some
logic change: e.g.  instead of defining `MG_ENABLE_MQTT`, one should
define `MIOT_ENABLE_MQTT`, which should imply `MG_ENABLE_MQTT` as well.

Macro refactoring is going to be done in another PR.

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It wasn't checked for `NULL`, and on CC3200 NULL dereferencing addresses
doesn't cause a crash, so it worked by pure luck: ctx->ssl_key was 0.
After `mg_` to `miot_` refactoring it's not the case anymore (presumably
because linker arranged objects in a different order), so this bug shown
up.

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in light of the root cause discovered by rojer
and addressed in cesanta/dev#5882 for the http connection code path.

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CC3200 (TI's C library) doesn't like it

https://e2e.ti.com/support/development_tools/compiler/f/343/t/554883

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Digital signature key usage is required for the key to be used for (EC)DHE.

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Currently only OpenSSL API impl, but the plan is to add more.

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