AOFSHRINK causes panic on server

[saques]

Describe the bug
Whenever we run an AOFSHRINK operation, the Tile38 server panics and crashes, causing our daemon to restart the process.

Expected behavior
AOFSHRINK ends and the server proceeds as normal.

Logs
We noted this log when using the -vv option:

2021/09/29 15:26:08 [INFO] aof shrink ended 742.016591ms
panic: interface conversion: interface {} is string, not *server.Hook

goroutine 3058 [running]:
github.com/tidwall/tile38/internal/server.byHookName({0xe77b80, 0xc043131f40}, {0xf6d080, 0xc050ee4000})
        /Users/josh/go/src/github.com/tidwall/tile38/internal/server/hooks.go:26 +0x85
github.com/tidwall/btree.(*node).find(0xc04fb24600, {0xe77b80, 0xc043131f40}, 0x10667f0, 0xc050305df8, 0x0)
        /Users/josh/go/src/github.com/tidwall/tile38/vendor/github.com/tidwall/btree/btree.go:119 +0x34a
github.com/tidwall/btree.(*BTree).GetHint(0xc000096540, {0xe77b80, 0xc043131f40}, 0x0)
        /Users/josh/go/src/github.com/tidwall/tile38/vendor/github.com/tidwall/btree/btree.go:344 +0x11f
github.com/tidwall/tile38/internal/server.(*Server).aofshrink.func2.4(0xc000226900, 0xc050305e50, 0xc050305e98, 0xc050305ec8)
        /Users/josh/go/src/github.com/tidwall/tile38/internal/server/aofshrink.go:184 +0xf1
github.com/tidwall/tile38/internal/server.(*Server).aofshrink.func2(0xc0002269f0)
        /Users/josh/go/src/github.com/tidwall/tile38/internal/server/aofshrink.go:218 +0x345
github.com/tidwall/tile38/internal/server.(*Server).aofshrink(0xc000226900)
        /Users/josh/go/src/github.com/tidwall/tile38/internal/server/aofshrink.go:304 +0x15b
created by github.com/tidwall/tile38/internal/server.(*Server).command
        /Users/josh/go/src/github.com/tidwall/tile38/internal/server/server.go:1131 +0x4f7d

Operating System (please complete the following information):

• OS: Linux (Ubuntu)
• Version: 18.04

[tidwall]

I just pushed a fix and a new release.