feat(core): use all available cores by default

[puzpuzpuz]

Changes default thread pool counts to make use of all available CPU cores instead of half of them as we do it now. Includes:

• Minimal size of shared worker thread pool is now 2. That's to avoid starvation (incl. health check) due to a long running query.
• A few spare cores are left for >8 and >16 cores cases. That's to leave some room to breathe to JVM & OS.

[puzpuzpuz]

cc @javier who also thinks we underutilize available resources with the default configuration.

[ideoma]

[PR Coverage check]

😍 pass : 4 / 6 (66.67%)

file detail

	path	covered line	new line	coverage
🔵	io/questdb/PropServerConfiguration.java	4	6	66.67%

[puzpuzpuz]

@amyshwang we need to update Capacity Planning page now. Is it ok if I update it or you'd prefer to do it yourself?