Block store (#578)

Contains the implementation of the block store

In this pr is a simple way to send and receive data through a database

I have implemented the base functionality as well as quite a few tests
to make sure that everything works
 
There are a few methods that have yet to be implemented, but
theoretically they should be implemented as calls to the other
functions, ie append should just be a call to WriteAt

This doesn't affect anything yet so it can safely be merged whenever. I
don't want this pr to stagnate like file view, so I'm happy to write
multiple prs for this

wavesrv/pkg/blockstore/schema.sql

@@ -0,0 +1,21 @@
+CREATE TABLE schema_migrations (version uint64,dirty bool);
+CREATE UNIQUE INDEX version_unique ON schema_migrations (version);
+CREATE TABLE block_file (
+    blockid varchar(36) NOT NULL,
+    name varchar(200) NOT NULL,
+    maxsize bigint NOT NULL,
+    circular boolean NOT NULL,
+    size bigint NOT NULL,
+    createdts bigint NOT NULL,
+    modts bigint NOT NULL,
+    meta json NOT NULL,
+    PRIMARY KEY (blockid, name)
+);
+
+CREATE TABLE block_data (
+    blockid varchar(36) NOT NULL,
+    name varchar(200) NOT NULL,
+    partidx int NOT NULL,
+    data blob NOT NULL,
+    PRIMARY KEY(blockid, name, partidx)
+);