WifiClient::write refactoring (second attempt)

[igrr]

Another attempt to do the same thing as in #1570.

WiFiClient::write used to block for every call until data was acknowledged by remote side. This also meant that for a series of WiFiClient::write or WiFiClient::print calls, each call would actually invoke tcp_output to send data and wait for acknowledgement, making the whole process really slow. Also with that design WiFiClient::setNoDelay didn't have any effect — because Nagle algorithm was never used in the first place.

With this change, WiFiClient::write will return as soon as all supplied data has been passed to the TCP stack. In practice that means that as much as 2 * 1460 bytes can be sent before WiFiClient::write will block. This change also changes WiFiClient::write behaviour for large blocks of data. Previously it would only write as much as TCP stack was willing to take (usually 2 * 1460 bytes) and not more. Now write method actually tries to send all data, i.e. if all data can not be sent at once, it will wait for more buffer space to be available and then send the rest. I'm also considering adding WiFiClient::setNonBlocking(bool) method to disable blocking altogether.

This change also removes all split-into-chunks-and-write code from ESP8266WebServer library because this is now handled by WiFiClient.

[codecov-io]

Current coverage is 27.61%

Merging #2177 into master will not change coverage

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