[#291] Make the version number comparable

[fluca1978]

Three new constants are introduced here:

• PGAGROAL_MAJOR_VERSION (number)
• PGAGROAL_MINOR_VERSION (number)
• PGAGROAL_PATCH_VERSION (number)
• PGAGROAL_VERSION (string)

that represent, as integer literals and string composition,
the parts of the version number.
The values are copied from the variables in the CmakeLists.txt file
that creates the building flow, so changing the values in a single
place changes and propagates the changes all over the source code.

A few utility functions have been introduced to compose the three
constant into a unique number that can be used to compare the version
currently running, so that features and/or message can be targeted as
conditionals.

Introduced functions are:

• pgagroal_version_as_number() returns an unique sortable version
  number. For example, version 1.5.0 is returned as 15000, version 1.6.2
  as 16002, and so on;
• pgagroal_version_number() returns the currently running version
  number, that is invokes pgagroal_version_as_number() with the
  predefined constants.

The pgagroal_version_as_number() accepts individual values, while
pgagroal_version_number() provides the currently running version
number. The idea is that, thanks to both the functions, it is possible
to check for a feature that depends on a version in a way like the
following one:

if (pgagroal_version_numer() >=
pgagroal_version_as_number(2,1,0))
// version is greater than 2.1.0 !

The utility function pgagroal_version_ge() does exactly the above:
it accepts the three parts of a version number and checks if the
currently running version is greater or equal (hence, 'ge') of the
specified triplet, so that the above piece of code becomes:

if (pgagroal_version_ge(2,1,0))
// version greater or equal 2.1.0

Close #292
See #253, #289, #290, #293

[fluca1978]

I force pushed the same commit with the correct refence to the issue, that is 292, not 291.

[jesperpedersen]

Nice !

[jesperpedersen]

Merged.

Thanks for your contribution !