Move oshdb-api's backend implementation into separate submodule(s)

[tyrasd]

Description

This creates a new submodules oshdb-api-ignite which contains the backend implementation code for running OSHDB queries on ignite.

I kept the JDBC/H2 backend implementation in the oshdb-api module for now, since it is a relatively lightweight (from a code and probably also from a dependencies point of view), and pulling it out would come with some annoyances: mainly that most tests (even the ones in ignite) use the H2 backend implementation as a "default" backend to run queries and/or to access keytables.

Some of the above points could be solved in the future (e.g. grid-included keytables, implementing a special in-memory "dummy" backend for tests, etc.). So, perhaps it's better to have a separate oshdb-api-jdbc (oshdb-api-h2) module already now? //edit: it would likely look more or less like this in at the moment: #388

PS: This also upgrades ignite to the newest version (2.10.0).

Corresponding issue

Closes #16 (?)
Closes #386

New or changed dependencies

• introduced new submodule oshdb-api-ignite
• moved ignite dependencies from oshdb-api to the new module

Checklist

• My code follows the code-style rules, and I have checked on the static analyses and benchmark (if applicable) results
• I have commented my code
• I have written javadoc (required for public classes and methods)
• I have added sufficient unit tests
• I have made corresponding changes to the documentation
• I have updated the CHANGELOG.md
• I have adjusted the examples or created an issue in the corresponding repository
• I have adjusted the benchmark or created an issue in the corresponding repository -> https://gitlab.gistools.geog.uni-heidelberg.de/giscience/big-data/ohsome/oshdb-benchmarks/-/blob/master/pom.xml

[joker234]

General question: do we plan to add more backends? If yes, we could use a sub-groupId like this:

<groupId>org.heigit.ohsome.oshdb-backend</groupId>
<artifactId>ignite-backend</artifactId>

[tyrasd]

General question: do we plan to add more backends? If yes, we could use a sub-groupId like this:

<groupId>org.heigit.ohsome.oshdb-backend</groupId>
<artifactId>ignite-backend</artifactId>

IMHO a realistic number would be that we eventually have maybe 3 or 4 actively maintained backend implementations. I think it's fine to keep the flat structure for now.