Change packaging strategy of quickfixj-messages-all to prevent VerifyError

In version 1.6.0, the message-all jar is made by merging the jar each individual FIX version.
An issue arises since the type of some fields may be change between FIX versions.

For instance, the field 43 ("PossDupFlag") is a String from Fix 4.0 to Fix 4.2 and becomes a Boolean in FIX 4.3.
So classes from the jar messages-fix42 and below expect to see a StringField for field 43, whereas classes from messages-fix43 and over expect a BooleanField.
When jars are merged, only one version of the PossDupFlag is kept, resulting in a VerifyError either in fix42 and below or fix43 and over classes (depending on whioch version of PossDup has been kept).
This also happens on several other fields.

This ticket aims at changing the packaging strategy by first merging the generated source code from all FIX specs, and then compiling instead of merging classes compiled individually for each FIX spec.
The new message-all is then consistent and compatible with the quickfixj-core module which uses the same approach.

An issue remains though as only one single version of the fields that change type (field 43 for example) is compiled.
By default, the most recent version of the field is kept
It seems that this conflict was handled the same before, it thus must not be that a big issue (to be confirmed)