Thread safety fix in logging and SetLevel

[yacovm]

https://github.com/op/go-logging/issues/104

When a logging is called concurrently with SetLevel, a panic occurs: fatal error: concurrent map read and map write

goroutine 20 [running]: runtime.throw(0x5f7cc0, 0x21) /home/yacovm/OBC/go/src/runtime/panic.go:530 +0x90 fp=0xc820038d58 sp=0xc820038d40 runtime.mapaccess2_faststr(0x551020, 0xc820078780, 0x5c9cc8, 0x4, 0x10, 0x551820) /home/yacovm/OBC/go/src/runtime/hashmap_fast.go:307 +0x5b fp=0xc820038db8 sp=0xc820038d58 _/home/yacovm/go-logging/go-logging.(_moduleLeveled).IsEnabledFor(0xc8200981e0, 0x4, 0x5c9cc8, 0x4, 0x0) /home/yacovm/go-logging/go-logging/level.go:110 +0x62 fp=0xc820038e18 sp=0xc820038db8 _/home/yacovm/go-logging/go-logging.(_Logger).IsEnabledFor(0xc8200788a0, 0x4, 0x0) /home/yacovm/go-logging/go-logging/logger.go:140 +0x4d fp=0xc820038e48 sp=0xc820038e18 _/home/yacovm/go-logging/go-logging.(_Logger).log(0xc8200788a0, 0x4, 0x0, 0xc8200d9b30, 0x1, 0x1) /home/yacovm/go-logging/go-logging/logger.go:144 +0x2f fp=0xc820038eb8 sp=0xc820038e48 _/home/yacovm/go-logging/go-logging.(_Logger).Info(0xc8200788a0, 0xc8200d9b30, 0x1, 0x1) /home/yacovm/go-logging/go-logging/logger.go:239 +0x51 fp=0xc820038ef0 sp=0xc820038eb8 _/home/yacovm/go-logging/go-logging.TestConcurrency(0xc8200a0120) /home/yacovm/go-logging/go-logging/logger_test.go:73 +0x11f fp=0xc820038f68 sp=0xc820038ef0 testing.tRunner(0xc8200a0120, 0x6b8468)

I added a RWLock to protect the map and added a regression test that many times fails without the locks