Let SQLite use pread/pwrite

With current settings SQLite was using lseek/read syscalls to read data, e.g.:

	20:43:17.640660 fcntl(3, F_SETLK, {l_type=F_UNLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0
	20:43:17.640683 fcntl(3, F_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=1073741824, l_len=1}) = 0
	20:43:17.640705 fcntl(3, F_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=1073741826, l_len=510}) = 0
	20:43:17.640725 fcntl(3, F_SETLK, {l_type=F_UNLCK, l_whence=SEEK_SET, l_start=1073741824, l_len=1}) = 0
	20:43:17.640744 stat(".../neo.sqlite-journal", 0x7ffef2c91080) = -1 ENOENT (No such file or directory)
	20:43:17.640764 lseek(3, 24, SEEK_SET)  = 24
	20:43:17.640779 read(3, "\0\0\0\33\0\0\10\235\0\0\10]\0\0\0\27", 16) = 16
	20:43:17.640795 stat(".../neo.sqlite-wal", 0x7ffef2c91080) = -1 ENOENT (No such file or directory)

but if we allow it to use pread it will be only 1 system call instead of 2 and
reading this way can also be done in parallel because there is no global to
file seeking:

	20:48:42.668466 fcntl(3, F_SETLK, {l_type=F_UNLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0
	20:48:42.668501 fcntl(3, F_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=1073741824, l_len=1}) = 0
	20:48:42.668522 fcntl(3, F_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=1073741826, l_len=510}) = 0
	20:48:42.668542 fcntl(3, F_SETLK, {l_type=F_UNLCK, l_whence=SEEK_SET, l_start=1073741824, l_len=1}) = 0
	20:48:42.668561 stat(".../neo.sqlite-journal", 0x7ffdbc1f22c0) = -1 ENOENT (No such file or directory)
	20:48:42.668580 pread64(3, "\0\0\0\33\0\0\10\235\0\0\10]\0\0\0\27", 16, 24) = 16
	20:48:42.668597 stat(".../neo.sqlite-wal", 0x7ffdbc1f22c0) = -1 ENOENT (No such file or directory)

(if needed this enablement can be done per OS)

Author: navytux

sqlite3.go

@@ -9,7 +9,7 @@ package sqlite3
 
 /*
 #cgo CFLAGS: -std=gnu99
-#cgo CFLAGS: -DSQLITE_ENABLE_RTREE -DSQLITE_THREADSAFE=1 -DHAVE_USLEEP=1
+#cgo CFLAGS: -DSQLITE_ENABLE_RTREE -DSQLITE_THREADSAFE=1 -DHAVE_USLEEP=1 -DHAVE_PREAD64=1 -DHAVE_PWRITE64=1
 #cgo CFLAGS: -DSQLITE_ENABLE_FTS3 -DSQLITE_ENABLE_FTS3_PARENTHESIS -DSQLITE_ENABLE_FTS4_UNICODE61
 #cgo CFLAGS: -DSQLITE_TRACE_SIZE_LIMIT=15
 #cgo CFLAGS: -DSQLITE_DISABLE_INTRINSIC