Convert CryptoToFiatConverter into a Singleton

[glonlas]

Summary

Speed up unit test from 60s to 6s by Change CryptoToFiatConverter() to a Singleton.

The issue is CryptoToFiatConverter() call Pymarketcap() lib at the __init__. When Pymarketcap() get instanced, it calls some other services. It results in a latency of 2 sec.

By changing CryptoToFiatConverter() to a Singleton, we avoid to init Pymarketcap() every time.

Solve the issue: #420

Quick changelog

• Change CryptoToFiatConverter() to a Singleton. The bot will instance Pymarketcap() only once.
• Accelerate Unit test
• Accelerate every time the bot calls CryptoToFiatConverter()

What's new?

Faster unit tests, Faster bot.
Below the new performance of the tests.

	Test	Original duration	New duration
[x]	freqtrade/tests/test_main.py::test_handle_trade_roi	6.70s	0.02s
[x]	freqtrade/tests/test_main.py::test_handle_overlpapping_signals	4.23s	0.02s
[x]	freqtrade/tests/test_main.py::test_sell_profit_only_enable_profit	4.15s	0.01s
[x]	freqtrade/tests/test_main.py::test_handle_trade_experimental	4.12s	0.01s
[x]	freqtrade/tests/test_main.py::test_sell_profit_only_disable_loss	4.10s	0.01s
[x]	freqtrade/tests/test_main.py::test_sell_profit_only_disable_profit	4.10s	0.02s
[x]	freqtrade/tests/test_main.py::test_process_trade_creation	2.92s	0.04s
[x]	freqtrade/tests/test_main.py::test_process_trade_handling	2.36s	0.02s
[x]	freqtrade/tests/test_main.py::test_sell_profit_only_enable_loss	2.29s	0.06s
[x]	freqtrade/tests/test_main.py::test_execute_sell_without_conf_sell_up	2.25s	0.01s
[x]	freqtrade/tests/test_main.py::test_close_trade	2.13s	0.02s
[x]	freqtrade/tests/rpc/test_rpc_telegram.py::test_status_handle	2.10s	0.01s
[x]	freqtrade/tests/rpc/test_rpc_telegram.py::test_count_handle	2.09s	0.01s
[x]	freqtrade/tests/rpc/test_rpc_telegram.py::test_status_table_handle	2.09s	0.01s
[x]	freqtrade/tests/test_main.py::test_execute_sell_without_conf_sell_down	2.07s	0.01s
[x]	freqtrade/tests/test_main.py::test_create_trade_minimal_amount	2.06s	0.01s
[x]	freqtrade/tests/rpc/test_rpc_telegram.py::test_performance_handle	2.03s	0.02s
[x]	freqtrade/tests/test_main.py::test_create_trade	2.02s	0.01s
[x]	freqtrade/tests/test_fiat_convert.py::test_fiat_convert_is_supported	1.25s	0.08s
[x]	freqtrade/tests/test_fiat_convert.py::test_fiat_convert_get_price	1.23s	0.08s
[x]	freqtrade/tests/test_fiat_convert.py::test_fiat_convert_find_price	1.23s	0.08s
[x]	freqtrade/tests/test_fiat_convert.py::test_fiat_convert_add_pair	1.19s	0.08s