I could honestly not make much sense of the timer implementation
that was here. Instead I've implemented the type of timer algorithm
that I've used before, and tested it instead.

The concept is extremely simple: all timers are put in an ordered
list. We check every server tick if any of the timers have
elapsed, and execute the function associated with this timer.

We know that many timers by themselves cause new timers to be
added to this list, so we iterate *backwards* over the timer
list. This means that new timers being added while timers are
being executed, can never be executed in the same function pass,
as they are always appended to the table *after* the end of
the table, which we will never reach in the current pass over
all the table elements.

We switch time keeping to minetest.get_us_time(). dtime is
likely unreliable and we have our own high-res timer that we
can fix if it is indeed broken. This removes the need to do
any sort of time keeping.

Add new node property 'floodable', default false
Define "air" as floodable = true in C++ and lua

Simplified the regex used, added comments and changed the error message
to contain the correct mod name.

The player name is now added in the field "dropped_by" on the created
entity.

Fixes #3280

The callback can now be invoked with either the player object or name as
the first parameter, and with either a table or a list of strings, like
this:

    minetest.check_player_privs(player_name, { shout = true, fly = true })
    minetest.check_player_privs(player_name, "shout", "fly")
    minetest.check_player_privs(player, { shout = true, fly = true })
    minetest.check_player_privs(player, "shout", "fly")

- Add warning log level
- Change debug_log_level setting to enumeration string
- Map Irrlicht log events to MT log events
- Encapsulate log_* functions and global variables into a class, Logger
- Unify dstream with standard logging mechanism
- Unify core.debug() with standard core.log() script API

1. Check for entity addition success in spawn_item implementation
2. Check for success in item_drop callback, so that the player
doesn't lose the item if they are outside bounds and try to drop it.
3. When existing player joins game, check that their position is inside
map bounds. If not, set their position to the return value of findSpawnPos().
4. Make findSpawnPos() respect the border

2 fixes a lua crash if a player drops an item outside map bounds.
3 fixes an assertion crash if a player leaves when being outside map bounds,
and then rejoins.

* abort if theres no active timer
* only reduce the timer.time of all timers when its necessary
* move updating timers_to_add into a seperate function

Now logs

ACTION[ServerThread]: player sets time to 6:03

instead of

ACTION[ServerThread]: player sets time to 6:3

  * The time command now accepts parameters in the form <hour>:<minute>,
    and if invoked with no parameters returns the current time in said format.

Items dropped into unloaded map space will crash game so here's a fix...

Don't ident too much, and add a comment.

On calling clear_redistered_biomes the registered_biomes table is cleared
by creating a new empty table, but the pointer is not updated to point to
the new one. So after calling more register_biome, the registered_biome
table always contains 0 items, which is an error. Instead, the table is
cleared by removing all its items so the pointer (minetest.registered_*)
remains valid.

Fix unused (and so, broken) enable_rollback_recording. This option must be reloaded at server loop but loaded when server starts, for data consistency (not a hot load variable)
ok @ShadowNinja