qt: Adjust BLOCK_CHAIN_SIZE to 20GB

This increases the space requirement for selecting a data directory in
the UI.
As suggested by @bardiharborow, fixes #3830.

Author: laanwj

src/qt/intro.cpp

@@ -14,7 +14,7 @@
 
 /* Minimum free space (in bytes) needed for data directory */
 static const uint64_t GB_BYTES = 1000000000LL;
-static const uint64_t BLOCK_CHAIN_SIZE = 10LL * GB_BYTES;
+static const uint64_t BLOCK_CHAIN_SIZE = 20LL * GB_BYTES;
 
 /* Check free space asynchronously to prevent hanging the UI thread.