Cache the relation map dictionary

[marpulli]

This speeds up my local test suite from 2 minutes to 30 seconds in a project that uses ormar.

This addresses an issue similar to this #957

[codspeed-hq]

CodSpeed Performance Report

Merging #1076 will degrade performances by 23.09%

_{Comparing marpulli:speed-up (b97f5a5) with master (1ed0d5a)}

Summary

⚡ 1 improvements
❌ 5 regressions
✅ 78 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	marpulli:speed-up	Change
⚡	test_count[1000]	25 ms	21.3 ms	+17.34%
❌	test_max[250]	10.4 ms	13.4 ms	-22.86%
❌	test_min[1000]	12.7 ms	16.6 ms	-23.09%
❌	test_first[250]	10.2 ms	12.9 ms	-20.91%
❌	test_get_or_create_when_get[250]	10.2 ms	13.2 ms	-22.93%
❌	test_get_or_none[250]	10.3 ms	13.2 ms	-21.66%