Fix for previous commit.

Add minetest.get_server_uptime() function to Lua API

Update lua_api.txt.

* Create UnitSAO, a common part between PlayerSAO & LuaEntitySAO
* Move breath to PlayerSAO & LocalPlayer
* Migrate m_yaw from (Remote)Player & LuaEntitySAO to UnitSAO
* Migrate m_yaw from Player to LocalPlayer for client
* Move some functions outside of player class to PlayerSAO/RemotePlayer or LocalPlayer depending on which class needs it
* Move pitch to LocalPlayer & PlayerSAO
* Move m_position from Player to LocalPlayer
* Move camera_barely_in_ceiling to LocalPlayer as it's used only there
* use PlayerSAO::m_base_position for Server side positions
* remove a unused variable
* ServerActiveObject::setPos now uses const ref
* use ServerEnv::loadPlayer unconditionnaly as it creates RemotePlayer only if it's not already loaded
* Move hp from Player to LocalPlayer
* Move m_hp from LuaEntitySAO to UnitSAO
* Use m_hp from PlayerSAO/UnitSAO instead of RemotePlayer

When rewriting keycode.cpp I overlooked this function, fixes #4683 completely

this should fix #4683

This commit modifies the liquid transforming procedure to light and
unlight nodes instead of whole map blocks.

This commit rewrites the procedure that is responsible for light
updating.

this commit
-provides iterative solutions for unlighting and light spreading
-introduces a new priority queue-like container for the iteration
-creates per-node MapBlock caching to reduce retrieving MapBlocks from
the map
-calculates with map block positions and in-block relative node
coordinates
-skips light updating if it is not necessary since the node's new light
will be the same as its old light was

This solves the problem whith building where build fails if the libaries have different names.

Flash alpha maximum is reduced from 180 to 127 to avoid player blindness
in combat. Flash alpha minimum is unchanged.
The 'damage_flash' value is now limited to max alpha, to avoid multiple
hits creating a huge value that causes flash to stay at maximum alpha
for a long period. Now alpha always starts to fade immediately after
taking damage.
Both problems can be seen in Minetest let's play videos.
Simplify and optimise some code.

Move static object storage force-delete message from errorstream to
warningstream.
Increase 'max objects per block' setting to 64.
Add missing spaces in warning code.

Rename "refresh" to "processInitData"

Modification of the emergeblocks internal state was not protected
by a lock, causing a race condition.
This can be reproduced by repeatedly running emergeblocks for an
already-generated section of the map (with multiple emerge threads).

	These methods do not actually fail but instead clear the item stack
	and return false if a value like e.g. "" is passed.