Host Keepalive configurable timeout with 2s default

[thinkyhead]

This PR makes it possible to configure the Host Keepalive timeout, and also to enable/disable the host Keepalive altogether. The following changes are made:

• Add a DEFAULT_KEEPALIVE_INTERVAL setting with a default value of 2
• Add a host_keepalive_interval variable to replace hardcoded interval
• Modify the host keepalive code to utilize the new variable
• Add a GCode M113 to get or set host_keepalive_interval
• Add host_keepalive_interval to EEPROM, bumping the version to "V24"

[jbrazio]

Should this be really stored on the EEPROM ?
Despite all the philosophical discussion about this feature it's not that important to change & remember the setting value at runtime, it's more of thing to be "set and forget" at compile time IMO.

[CONSULitAS]

As i would understand it, the default is for using on Terminal.

If a host needs faster or slower response it should set while connecting and reset on disconnect.

So @repetier and @foosel could have an own (even stored) parameter for this and tune it for their side of the protocol.

[thinkyhead]

@jbrazio The reason this setting is was being added to EEPROM is because it is a configurable setting that can be overridden. Usually, when there's a setting like this where the Configuration value is only supplying a default for something which can be changed later, we also save it in EEPROM and include it in the M503 output. (In any case, this is why the setting is called DEFAULT_KEEPALIVE_INTERVAL — to indicate it can be changed as a variable.)

Anyway, if it doesn't seem too vital to save this in EEPROM, then it's no big deal for me to remove that part of this PR.

[jbrazio]

I like the possibility to tweak during runtime the value (this was a great idea), but who may tweak this ? Developers and Host software which IMO is highly "session" dependent thus not really requiring to be remembered between power cycles.

[thinkyhead]

@repetier @foosel @kliment

Ok, hostie people! Please give this change a try. If a 2s timeout produces too many messages for your taste (as it will with G29, of course) then perhaps we can try a slightly longer default interval (like 5s).